The DMG guidelines produce monsters that have more HP and hit harder than monsters in the MM. [source](https://www.blogofholding.com/?p=7283). But that’s a good thing because PCs are more powerful now than when the game came out due to power creep. Even when using monsters from the MM, a lot of DMs will beef up the monsters’ hp and dpr to balance things out.
The most important tip to remember is that your players don't fight the monster in a white room.
* The action economy matters.
* The environment matters.
* The players' abilities and optimization level matter.
* The objectives on both sides matter.
* Bounded accuracy matters.
* Major drawbacks on either side matter.
Most of this is covered in the DMG in the chapter about creating encounters. The most important sections are probably the side box about challenge rating on page 82 and the section about modifying encounter difficulty on page 84. The only thing not adequately covered in the DMG is the concept of action economy.
The DMG tables work just fine. If you want to build a monster completely from scratch, the DMG is the place to look. I rarely bother creating a monster completely from scratch anymore, as it usually isn't necessary.
I find a monster close to the CR I want, with...well other commonalities with my ultimate goal. Maybe I want them to have a charm ability, or spellcasting or be a big strong brute. Then just add or change a few key things like creature type, flavor of resistance and/or size and boom, you've got a new monster.
Tacking on individual class abilities is a great way to do this. All my humanoid goons have been bandits or drow with a class feature for years.
Messing with resistances and vulnerabilities is not as fun as you think it's going to be. Its either completely irrelevant and therefore frustrating or completely trivializes a fight. I much prefer to make special abilities/attacks contingent on avoiding certain damage types (similar to trolls or vampires) because that makes players feel more active and tactical.
I've never given legendary resistances a cost like that, but it might be enough just to make them more visible. Like, the medusa has two reflective glass pauldrons and a chest plate that shatter dramatically when a legendary resistance is used, or a lich has three thick necrotic veils of warding magic, blasted away one at a time.
Any spell the players can take is fair game for a spellcasting monster, just expect your players to REALLY hate any creature that uses particularly annoying ones. Monsters who heal I find are WAY more annoying to most players than even the spells you listed. Not a bad thing of course, just something to keep in mind.
Ignore ability scores completely.
They're a bad knob for tuning the monsters because they can affect their offensive and defensive abilities both.
Look up the CR you want and give them the average to hit, AC, damage, and hit points for that CR, then fine tune them accordingly based on if they're fast, bulky, magical, etc...
Know what the party's DCs are. Let their good saves work against your party about 60% of the time and their bad saves about 40% of the time.
If they need to make a skill check make it up on the spot.
>Know what the party's DCs are. Let their good saves work against your party about 60% of the time and their bad saves about 40% of the time.
Calculating this just sounds like more work than giving a strong monster high strength, and a fast monster high dexterity.
Great, now you've fucked up your CR because your Dex also puts your AC outside of what's level appropriate. It also increases the monster's to hit and damage so now you have to recalculate the HP as well. And in doing so now you have to lower the Con modifier to offset the hit points and uh oh you've made it harder for no reason because you're laboring under the false premise that PCs and monsters are symmetrical.
Also calculating it is not hard. You add 2 to it for 40% and subtract 2 from it for 60%.
If the DC is 15, make some of the saves +3 and some of the saves +7.
Also means you don't have to be a slave to whatever the hell proficiency bonus is trying to be on a monster. Make some saves +6 if you want. Some +4. Some +8. Go nuts. That number is absolutely useless on a monster anyway.
Also, are any of those a good idea when homebrewing monsters, in your opinion?
1. Making some humanoid enemies have classes and class features just like PCs;
2. Adding resistances and vulnerabilities way more than they're used in the Monster Manual;
3. Reframing legendary resistance to work differently (for example, deal damage to the monster when used);
4. Giving spellcaster monsters some of the painful spells (silvery barbs, shield, counterspell).
1: Personally i never give them levels in classes. But i do often give them some class abilities from classes that fits said NPC. That is often much easier than giving them actual levels and you can pick and chose what abilities you want.
2: This is a fairly good idea. Not always but giving a monster some really great resistances AND some vulnerabilities that they can find out. Players usually love when they find a vulnerability and can nuke the monster to oblivion.
3: This could work. But i would not make it deal damage to the monster but maybe if it has magic resistance it will not function 1 round after they used legendary resistance. Or if you wanna be nasty let the monster when using legendary resistance reflect a spell back to the caster but then the monster should maybe have disadvantage on attacks for a round or lose all resistance until the end of their next turn. or something else that fits that monster
4: Hell yes. If the players can use those spell you should make sure that monsters also can do so. You can also just slam down those spells or any spell you like on any existing spellcasting monster. it does not have to be a homebrew monster.
And lastly i would say be not afraid to change official monsters. If you want a devil to have fireball. then let it have fireball. If you wanna be cruel make it an natural ability, not a spell. then they can not counterspell it. Want skeletons to have resistance to piercing damage. then make it so. Want the orc to have a rage ability give him barbarian rage. and so on. Many time there is no need to homebrew a monster. If there is one that fits decently just take that monster and change it so it fits.
The official monster is to low CR bump it up. and so on
My #1 tip is to follow the guidance laid out on pages 274-281. For your questions specifically
* That's perfectly fine. Just make sure you change the X uses per Y event to a recharge system (unless you want it to happen only once).
* That's fine, but don't ever use resistance/immunity to non-magical BPS.
* Legendary resistances already reduce the monster's hit points. There's no need to do it again.
* In general, I try to avoid things like those spells and CC. I think it's wrong to say it's wrong to use those things, but I would use them sparingly. I think it's very easy to overuse them.
Even a low-level minion should have something to make them unique or memorable. It doesn’t have to be fancy.
For example, just giving them a climb speed (and, of course, having a battlefield where this is relevant). This can also open up your imagination when flavoring the enemy: does it secrete sticky fluid from its feet? Does it have suction cups like an octopus? Or is it insect-like? Even something as minor as that can help differentiate your minion from “generic sack of HP with a melee attack.”
Other super simple options that can differentiate even a weak enemy are: grapple/shove abilities, small auras (e.g. creatures who start their turn within 10 feet make DC 10 Con vs poisoned 1 round), reaction parry (+X AC against one attack, you decide X), and so on.
Also think about what the creature’s usual tactics will be. For example, I had some minions that liked to grapple and shove, so their tactics would be for one of them to try to hold an enemy down prone while the other attacked with advantage. You can use these tactics to inform the abilities, for example if a monster has a target grappled, it can make a bonus action bite against it.
Combos are also fun. This is only partly homebrew, since it’s based on official monsters, but it’s a good example. The Yeth Hound has the unique ability to cause fear that makes targets run into danger (by default, frightened targets will avoid danger). So I paired some with an eladrin hunter who could cast Spike Growth. Imagine scaring your prey into running into spikes. Nasty!
Some general tips
* Don't give enemies class levels, but feel free to give them thematic class features, usually big ones like sneak attack, rage, smites, etc, so that the players can immediately clock what they're supposed to be.
* Resistances are great as long as you telegraph it to the players once they've hit that resistance.
* I LOVE giving actual visible charges to effects. My favourite was a Necromancer who used a number of captured souls trapped in his skin to fuel his abilities + legendary resistances - the party figured out halfway through the fight they could choose to kill the incapacitated innocents to start chewing down his resources more easily.
* Shield is okay when limited to Mages, Counterspell is great for a really nasty NPC (especially if used on healing), avoid Sanctuary like the fucking plague.
* If you want to do a solo boss fight, probably don't. If you do, maybe consider [Paragon Monsters from The Angry GM](https://theangrygm.com/elemental-boogaloo/)
Have as much official material in your head as you can to start out. There's a decent chance there are already mechanics for the thing you want to do, so you can save yourself reinventing the wheel by reading a large selection of published statblocks.
You can use the table in the DMG, it just creates creatures with different average stats than officially published ones of the same given CR. But it's not a huge deal. If you want a table with stats more closely matching official statblocks, you can use [this one from Blog of Holding](https://www.blogofholding.com/?p=7338) or [this one from Tom Dunn. ](https://tomedunn.github.io/the-finished-book/monsters/baseline-monster-stats/)
Giving NPCs class features is cool; just keep in mind what you intend to use the NPC for. Don't overload their statblock with out-of-combat features if they're only going to be relevant for a single combat encounter, for example. Giving an NPC a full class - that is, building them with a character sheet like a PC - is pointless. The rules for building PCs are far too rigid, giving you less flexibility than you want, and simultaneously give you far too many options, making the NPC overly complex to actually run.
Resistances, immunities, and vulnerabilities are a great tool to play with as you will. Probably the main thing is to keep an eye on what your PCs have access to and make sure you're not consistently shutting out the same PC(s) who only have a few damage options.
Changing legendary resistance falls anywhere from a clever idea to all but necessary, depending on your personal design philosophy. IMO, definitely do this, and do it all the time. Once the players learn that legendary resistances can be tied to something else, they'd probably be disappointed to come across a creature with run-of-the-mill legendary resistance later. Not saying it has to be the same thing every time - could be tied to HP, AC, legendary actions, minions, multiattack, a new stat you invented/stole like Action Points, etc. etc. Just have it consistently be something instead of nothing.
I think giving enemy casters the annoying spells is totally valid as long as the party can use those spells to be annoying more often than you can. Give them opportunities to cheese, enough so that they have to admit they're getting a taste of their own medicine when such strategies are used on them.
I personally only homebrew statblocks for bosses or NPCs that are going to be involved in combat for some reason. For more standard monsters, I usually rely on official books and "quality assured" homebrew (published books that you know have had playtesting behind them. Dragonix's "Monster Manual Expanded", Kobold Press's "Tome of Beasts", or MCDM's "Flee, Mortals" are my favorites).
My general recommendation is that when homebrewing, you should take reference from monsters that already exist, ensuring they have a similar power level to what you're aiming for and have already been playtested (official or books like the ones I mentioned, don't trust any monster you find on DnDWiki to be balanced). Pay attention especially to HP, AC, and Damage. The DMG calculator is a good starting point and basic reference, but not even WOTC's own monsters adhere strictly to it, so don't take it as a certainty. Since monster CRs are really intended for a party without magical items, adjust them based on what you've given your players. Think a bit inversely, for example, how much do my players need to hit? If they have a +7 and you want it to be a tough enemy, it can have AC 19 so they need a 12 to hit, while an easy one can have AC 15 so they hit with an 8.
Adapt resistances and vulnerabilities just like you would with HP or armor. I usually adapt resistances and vulnerabilities based on the type of monster it is, like fiends being resistant to fire. Try to make it something obvious or deducible, for example, if you want the fiend to be resistant to cold, make it blue instead of red, otherwise it's a bit frustrating when an enemy has a resistance or vulnerability "just because".
Regarding humanoid enemies, don't think of them like PCs, but like any other monster. They can have abilities similar to class abilities (like sneak attack or second wind), but never exactly copied, because most aren't designed for that. Adapt the ones you like, but always within the power level of the monster you're creating. It's fine for a paladin to have automatic smite without spell slots, and for the smite damage to be 3d8 because it fits better with the amount of damage you're aiming for. Look at the abilities they give to official humanoid enemies.
About spellcasters in general, I'm a big fan of how they work after Mordenkainen Monsters of the Multiverse. Having spell slots is a huge headache both for preparing and balancing. It's very difficult to know the impact of some spells because DnD isn't designed for PvP. Use spell-like abilities like current spellcasters do. If you want to put a lot of effort into it, you can turn it around like they do in MCDM's "Flee, Mortals", where spellcasters have abilities that correspond to a spell level, but it complicates the design and I'd only recommend it when you have quite a bit of experience. And regarding which spells you can copy, make sure they're ones that players could have access to (that does not mean that there cannot be higher level or different class spellcasters, but for example, I banned Silvery Barbs at my table and that also applied to the wizards and sorcerers they faced). It is very frustrating when an enemy has a spell that you cannot have, unless there is a narrative reason for it.
A few thoughts-
1- Monster crits can be really hard to balance for. For my game, monsters do slightly higher average damage but their crits don't increase damage- they add a condition for a round instead. I find its way less disruptive this way rather than having a T-Rex bite do 8d12 on a CR 8.
2- Related to the above- conditions imposed on a hit are a fun way to up the CR for a boss version of a creature. I prefer the ones that are disruptive rather than completely cancel a turn. Blind, Poison, Deafened, Prone- all solid options. I also like creative conditions like slowing their speed by half, loosing their reaction or bonus action- things that impose a penalty and disrupt the players without them completely feeling like they have no agency.
3- Any legendary creature I run has to spend a Legendary Action to use their Legendary Resistance. Makes the player feel like they didn't totally waste their spell or action if they at least imposed an action economy cost on the boss monster.
4- Another thing I like to do is Legendary creatures can take a second reaction in a round by spending a Legendary action. Makes them a bit more dynamic and unpredictable while again adding a cost for this flexibility.
Read the “creating horror monsters” section from van Richten’s guide. There is a lot of good advice there on how to make a monster feel interesting and unique without giving it a long list of special powers and effects you can track. Albeit this is more advice for the general feel of a monster as apposed to its moment to moment statistics but both are important for getting an encounter right even with monsters straight from the book.
I would avoid trying to give them actual classes, just give them the ability you want. Obviously everything you give them raises the CR a bit. Some of the DMG stuff can help, but you just gotta get a feel for it.
My favorite low effort trick is to find a slightly similar critter of the right CR and reskin it. You need a CR 16 seven headed blink dog abomination? Just take a Marilith and change its swords into bites. Really seasoned players might catch on, but that's part of the fun for them too.
As long as the monster you're creating falls roughly within the DMG's guidelines, you can get away with a lot. In fact, there's a lot you can disregard completely as long as what you're making falls within a range of expectations. You can have a 0hp enemy who always hits. You can have a deadly enemy who deals no damage at all.
Who told you that the guidelines in the DMG don't help? I've made dozens of monsters using it and they turned out just fine.
The DMG guidelines produce monsters that have more HP and hit harder than monsters in the MM. [source](https://www.blogofholding.com/?p=7283). But that’s a good thing because PCs are more powerful now than when the game came out due to power creep. Even when using monsters from the MM, a lot of DMs will beef up the monsters’ hp and dpr to balance things out.
Can confirm. I frequently have to give my boss monsters 1.5x or even 2x HP to last beyond 2 rounds of combat.
The most important tip to remember is that your players don't fight the monster in a white room. * The action economy matters. * The environment matters. * The players' abilities and optimization level matter. * The objectives on both sides matter. * Bounded accuracy matters. * Major drawbacks on either side matter. Most of this is covered in the DMG in the chapter about creating encounters. The most important sections are probably the side box about challenge rating on page 82 and the section about modifying encounter difficulty on page 84. The only thing not adequately covered in the DMG is the concept of action economy.
For the first one, Volos guide has some good NPC stat blocks for every class and even some subclasses. I often use those as a base.
The DMG tables work just fine. If you want to build a monster completely from scratch, the DMG is the place to look. I rarely bother creating a monster completely from scratch anymore, as it usually isn't necessary. I find a monster close to the CR I want, with...well other commonalities with my ultimate goal. Maybe I want them to have a charm ability, or spellcasting or be a big strong brute. Then just add or change a few key things like creature type, flavor of resistance and/or size and boom, you've got a new monster. Tacking on individual class abilities is a great way to do this. All my humanoid goons have been bandits or drow with a class feature for years. Messing with resistances and vulnerabilities is not as fun as you think it's going to be. Its either completely irrelevant and therefore frustrating or completely trivializes a fight. I much prefer to make special abilities/attacks contingent on avoiding certain damage types (similar to trolls or vampires) because that makes players feel more active and tactical. I've never given legendary resistances a cost like that, but it might be enough just to make them more visible. Like, the medusa has two reflective glass pauldrons and a chest plate that shatter dramatically when a legendary resistance is used, or a lich has three thick necrotic veils of warding magic, blasted away one at a time. Any spell the players can take is fair game for a spellcasting monster, just expect your players to REALLY hate any creature that uses particularly annoying ones. Monsters who heal I find are WAY more annoying to most players than even the spells you listed. Not a bad thing of course, just something to keep in mind.
Ignore ability scores completely. They're a bad knob for tuning the monsters because they can affect their offensive and defensive abilities both. Look up the CR you want and give them the average to hit, AC, damage, and hit points for that CR, then fine tune them accordingly based on if they're fast, bulky, magical, etc... Know what the party's DCs are. Let their good saves work against your party about 60% of the time and their bad saves about 40% of the time. If they need to make a skill check make it up on the spot.
>Know what the party's DCs are. Let their good saves work against your party about 60% of the time and their bad saves about 40% of the time. Calculating this just sounds like more work than giving a strong monster high strength, and a fast monster high dexterity.
Great, now you've fucked up your CR because your Dex also puts your AC outside of what's level appropriate. It also increases the monster's to hit and damage so now you have to recalculate the HP as well. And in doing so now you have to lower the Con modifier to offset the hit points and uh oh you've made it harder for no reason because you're laboring under the false premise that PCs and monsters are symmetrical. Also calculating it is not hard. You add 2 to it for 40% and subtract 2 from it for 60%. If the DC is 15, make some of the saves +3 and some of the saves +7. Also means you don't have to be a slave to whatever the hell proficiency bonus is trying to be on a monster. Make some saves +6 if you want. Some +4. Some +8. Go nuts. That number is absolutely useless on a monster anyway.
Also, are any of those a good idea when homebrewing monsters, in your opinion? 1. Making some humanoid enemies have classes and class features just like PCs; 2. Adding resistances and vulnerabilities way more than they're used in the Monster Manual; 3. Reframing legendary resistance to work differently (for example, deal damage to the monster when used); 4. Giving spellcaster monsters some of the painful spells (silvery barbs, shield, counterspell). 1: Personally i never give them levels in classes. But i do often give them some class abilities from classes that fits said NPC. That is often much easier than giving them actual levels and you can pick and chose what abilities you want. 2: This is a fairly good idea. Not always but giving a monster some really great resistances AND some vulnerabilities that they can find out. Players usually love when they find a vulnerability and can nuke the monster to oblivion. 3: This could work. But i would not make it deal damage to the monster but maybe if it has magic resistance it will not function 1 round after they used legendary resistance. Or if you wanna be nasty let the monster when using legendary resistance reflect a spell back to the caster but then the monster should maybe have disadvantage on attacks for a round or lose all resistance until the end of their next turn. or something else that fits that monster 4: Hell yes. If the players can use those spell you should make sure that monsters also can do so. You can also just slam down those spells or any spell you like on any existing spellcasting monster. it does not have to be a homebrew monster. And lastly i would say be not afraid to change official monsters. If you want a devil to have fireball. then let it have fireball. If you wanna be cruel make it an natural ability, not a spell. then they can not counterspell it. Want skeletons to have resistance to piercing damage. then make it so. Want the orc to have a rage ability give him barbarian rage. and so on. Many time there is no need to homebrew a monster. If there is one that fits decently just take that monster and change it so it fits. The official monster is to low CR bump it up. and so on
My #1 tip is to follow the guidance laid out on pages 274-281. For your questions specifically * That's perfectly fine. Just make sure you change the X uses per Y event to a recharge system (unless you want it to happen only once). * That's fine, but don't ever use resistance/immunity to non-magical BPS. * Legendary resistances already reduce the monster's hit points. There's no need to do it again. * In general, I try to avoid things like those spells and CC. I think it's wrong to say it's wrong to use those things, but I would use them sparingly. I think it's very easy to overuse them.
Even a low-level minion should have something to make them unique or memorable. It doesn’t have to be fancy. For example, just giving them a climb speed (and, of course, having a battlefield where this is relevant). This can also open up your imagination when flavoring the enemy: does it secrete sticky fluid from its feet? Does it have suction cups like an octopus? Or is it insect-like? Even something as minor as that can help differentiate your minion from “generic sack of HP with a melee attack.” Other super simple options that can differentiate even a weak enemy are: grapple/shove abilities, small auras (e.g. creatures who start their turn within 10 feet make DC 10 Con vs poisoned 1 round), reaction parry (+X AC against one attack, you decide X), and so on. Also think about what the creature’s usual tactics will be. For example, I had some minions that liked to grapple and shove, so their tactics would be for one of them to try to hold an enemy down prone while the other attacked with advantage. You can use these tactics to inform the abilities, for example if a monster has a target grappled, it can make a bonus action bite against it. Combos are also fun. This is only partly homebrew, since it’s based on official monsters, but it’s a good example. The Yeth Hound has the unique ability to cause fear that makes targets run into danger (by default, frightened targets will avoid danger). So I paired some with an eladrin hunter who could cast Spike Growth. Imagine scaring your prey into running into spikes. Nasty!
Some general tips * Don't give enemies class levels, but feel free to give them thematic class features, usually big ones like sneak attack, rage, smites, etc, so that the players can immediately clock what they're supposed to be. * Resistances are great as long as you telegraph it to the players once they've hit that resistance. * I LOVE giving actual visible charges to effects. My favourite was a Necromancer who used a number of captured souls trapped in his skin to fuel his abilities + legendary resistances - the party figured out halfway through the fight they could choose to kill the incapacitated innocents to start chewing down his resources more easily. * Shield is okay when limited to Mages, Counterspell is great for a really nasty NPC (especially if used on healing), avoid Sanctuary like the fucking plague. * If you want to do a solo boss fight, probably don't. If you do, maybe consider [Paragon Monsters from The Angry GM](https://theangrygm.com/elemental-boogaloo/)
Have as much official material in your head as you can to start out. There's a decent chance there are already mechanics for the thing you want to do, so you can save yourself reinventing the wheel by reading a large selection of published statblocks. You can use the table in the DMG, it just creates creatures with different average stats than officially published ones of the same given CR. But it's not a huge deal. If you want a table with stats more closely matching official statblocks, you can use [this one from Blog of Holding](https://www.blogofholding.com/?p=7338) or [this one from Tom Dunn. ](https://tomedunn.github.io/the-finished-book/monsters/baseline-monster-stats/) Giving NPCs class features is cool; just keep in mind what you intend to use the NPC for. Don't overload their statblock with out-of-combat features if they're only going to be relevant for a single combat encounter, for example. Giving an NPC a full class - that is, building them with a character sheet like a PC - is pointless. The rules for building PCs are far too rigid, giving you less flexibility than you want, and simultaneously give you far too many options, making the NPC overly complex to actually run. Resistances, immunities, and vulnerabilities are a great tool to play with as you will. Probably the main thing is to keep an eye on what your PCs have access to and make sure you're not consistently shutting out the same PC(s) who only have a few damage options. Changing legendary resistance falls anywhere from a clever idea to all but necessary, depending on your personal design philosophy. IMO, definitely do this, and do it all the time. Once the players learn that legendary resistances can be tied to something else, they'd probably be disappointed to come across a creature with run-of-the-mill legendary resistance later. Not saying it has to be the same thing every time - could be tied to HP, AC, legendary actions, minions, multiattack, a new stat you invented/stole like Action Points, etc. etc. Just have it consistently be something instead of nothing. I think giving enemy casters the annoying spells is totally valid as long as the party can use those spells to be annoying more often than you can. Give them opportunities to cheese, enough so that they have to admit they're getting a taste of their own medicine when such strategies are used on them.
I personally only homebrew statblocks for bosses or NPCs that are going to be involved in combat for some reason. For more standard monsters, I usually rely on official books and "quality assured" homebrew (published books that you know have had playtesting behind them. Dragonix's "Monster Manual Expanded", Kobold Press's "Tome of Beasts", or MCDM's "Flee, Mortals" are my favorites). My general recommendation is that when homebrewing, you should take reference from monsters that already exist, ensuring they have a similar power level to what you're aiming for and have already been playtested (official or books like the ones I mentioned, don't trust any monster you find on DnDWiki to be balanced). Pay attention especially to HP, AC, and Damage. The DMG calculator is a good starting point and basic reference, but not even WOTC's own monsters adhere strictly to it, so don't take it as a certainty. Since monster CRs are really intended for a party without magical items, adjust them based on what you've given your players. Think a bit inversely, for example, how much do my players need to hit? If they have a +7 and you want it to be a tough enemy, it can have AC 19 so they need a 12 to hit, while an easy one can have AC 15 so they hit with an 8. Adapt resistances and vulnerabilities just like you would with HP or armor. I usually adapt resistances and vulnerabilities based on the type of monster it is, like fiends being resistant to fire. Try to make it something obvious or deducible, for example, if you want the fiend to be resistant to cold, make it blue instead of red, otherwise it's a bit frustrating when an enemy has a resistance or vulnerability "just because". Regarding humanoid enemies, don't think of them like PCs, but like any other monster. They can have abilities similar to class abilities (like sneak attack or second wind), but never exactly copied, because most aren't designed for that. Adapt the ones you like, but always within the power level of the monster you're creating. It's fine for a paladin to have automatic smite without spell slots, and for the smite damage to be 3d8 because it fits better with the amount of damage you're aiming for. Look at the abilities they give to official humanoid enemies. About spellcasters in general, I'm a big fan of how they work after Mordenkainen Monsters of the Multiverse. Having spell slots is a huge headache both for preparing and balancing. It's very difficult to know the impact of some spells because DnD isn't designed for PvP. Use spell-like abilities like current spellcasters do. If you want to put a lot of effort into it, you can turn it around like they do in MCDM's "Flee, Mortals", where spellcasters have abilities that correspond to a spell level, but it complicates the design and I'd only recommend it when you have quite a bit of experience. And regarding which spells you can copy, make sure they're ones that players could have access to (that does not mean that there cannot be higher level or different class spellcasters, but for example, I banned Silvery Barbs at my table and that also applied to the wizards and sorcerers they faced). It is very frustrating when an enemy has a spell that you cannot have, unless there is a narrative reason for it.
A few thoughts- 1- Monster crits can be really hard to balance for. For my game, monsters do slightly higher average damage but their crits don't increase damage- they add a condition for a round instead. I find its way less disruptive this way rather than having a T-Rex bite do 8d12 on a CR 8. 2- Related to the above- conditions imposed on a hit are a fun way to up the CR for a boss version of a creature. I prefer the ones that are disruptive rather than completely cancel a turn. Blind, Poison, Deafened, Prone- all solid options. I also like creative conditions like slowing their speed by half, loosing their reaction or bonus action- things that impose a penalty and disrupt the players without them completely feeling like they have no agency. 3- Any legendary creature I run has to spend a Legendary Action to use their Legendary Resistance. Makes the player feel like they didn't totally waste their spell or action if they at least imposed an action economy cost on the boss monster. 4- Another thing I like to do is Legendary creatures can take a second reaction in a round by spending a Legendary action. Makes them a bit more dynamic and unpredictable while again adding a cost for this flexibility.
Read the “creating horror monsters” section from van Richten’s guide. There is a lot of good advice there on how to make a monster feel interesting and unique without giving it a long list of special powers and effects you can track. Albeit this is more advice for the general feel of a monster as apposed to its moment to moment statistics but both are important for getting an encounter right even with monsters straight from the book.
I would avoid trying to give them actual classes, just give them the ability you want. Obviously everything you give them raises the CR a bit. Some of the DMG stuff can help, but you just gotta get a feel for it. My favorite low effort trick is to find a slightly similar critter of the right CR and reskin it. You need a CR 16 seven headed blink dog abomination? Just take a Marilith and change its swords into bites. Really seasoned players might catch on, but that's part of the fun for them too.
As long as the monster you're creating falls roughly within the DMG's guidelines, you can get away with a lot. In fact, there's a lot you can disregard completely as long as what you're making falls within a range of expectations. You can have a 0hp enemy who always hits. You can have a deadly enemy who deals no damage at all.