The BBEG Fallacy
So... Let's talk for a moment or three about some of the structural underpinnings of D&D. Basically, in higher powered play, or just at higher levels, the only meaningful economy is economy of actions. Entire books have been written about it at my home on WotCO 339, so I'll spare you the long talk. Suffice to say that the average character can cast 2 or at most 3 spells in a round, before the application of significant optimization kung-fu.
So you have a party with two casters, and two warriors.
And a BBEG with three mooks.
So far, so good.
Except that the mooks are gonna need to close to melee, leaving them a standard action to use for combat at best. Worse, the villain is going to need to deal with the buffs and prep that the team has taken time to lay out. They may know more about him than he does about them. This means he may be busy fighting reactively, and unable to support his mooks with battlefield control or debuffs. War Weavers and other interesting classes can help fix this, but it's a core problem in the fact that D&D has uneven valuation for certain kinds of actions.
In other words, he has to fight you. You just need to kill him.
You have 6 spells to his three, and two full attacks on top of that. It's even worse if one of the melee roles is being filled by a gish build.
So you think, that's not a problem, I'll be fine with just upping his class levels. Well that may work, or more likely it will either fail or result in a TpK. After level six, each level a character has in a player class starts to matter more and more. This leads to bad places.
You need to prepare your BBEG to deal with more spells coming in than he can put out. Or you need to provide him with more caster support...
Suddenly this isn't about the BBeG anymore.
And that's fine.
In fact, that's probably for the best.
Why?
Combat in D&D is brutal, and unless you fudge dice, a really powerful villain is either going to die rapidly or cause some very serious and irreparable damage to the party. If that's okay, go back to the first post, and consider using one of those villains. Or go down to the next post and start thinking about how a tactically minded villain can mitigate these problems. Forgive my brevity.