Using Gestalts to create boss enemies

[Protato]

Has anyone ever made bosses or other NPCs as PC gestalts? For example, for a final boss that's in my head, I intend my villain to be someone who's fountain of youth is running out and needs a new source of immortality, and the party possibly knows how to get it. I don't know what monsters I'd use to represent him, and while I could homebrew one, at that point I'd probably just make a gestalt PC anyhow. For his class, I'm thinking of Wizard Evocation (or necromancer for flavor) and Warlock, possibly with two actions to make the game fairer for him, as I have six PCs and even if the boss is powerful and has backups that might still be an uphill battle.

Is this an okay or even workable idea? If so, does anyone have advice on doing this?

[JNAProductions]

It'd work... Okayish? Players tend to have high offensive and low defensive CRs, relatively speaking, but even then, most PCs of level X are CR X/2 or 2X/3, somewhere in that zone.

I'd honestly just stat this boss out as a monster-don't fiddle around with PC nonsense if you don't have to. Now, by all means, TAKE PC ABILITIES, but don't feel beholden to the classes.

[PhoenixPhyre]

It'd work... Okayish? Players tend to have high offensive and low defensive CRs, relatively speaking, but even then, most PCs of level X are CR X/2 or 2X/3, somewhere in that zone.

I'd honestly just stat this boss out as a monster-don't fiddle around with PC nonsense if you don't have to. Now, by all means, TAKE PC ABILITIES, but don't feel beholden to the classes.

Exactly. 5e PCs make poor monsters and worse bosses. Especially solo. You really really really really need legendary actions and (at higher levels) legendary resistances for a good boss fight and PC classes can't get those. And if you push the level (HP/damage) high enough to survive the opening nova on a low initiative, you're risking rocket tag. Not only that, most of the parts of a PC build are irrelevant to NPCs--all the resource recovery parts? The boss is only going to be fighting once that day. All those ribbons? Will they ever come up or can they just be understood?

Just go ahead and choose the HP/etc you need. Then add on the interesting parts of the classes you want. And you can pick and choose--you can have a person using wizard spells with warlock sub-class features. You can have a raging spell-caster (since you don't have to copy things word for word). In my book, it's more useful to be evocative of PC abilities than to duplicate PC builds.

[Unoriginal]

For 6 PCs, you're going to need Lair Actions and Legendary Actions, probably. Unless you give the bad guy enough mooks.

Otherwise I agree with what the others have said. Taking a look at the NPC statblocks for spellcasters would be wise