3.5 Badguy concept help

[No brains]

Dispel Magic - What Buffs?
Antimagic field - What spells?
Attacks of Opportunity against the spell caster - if you cannot concentrate then you lose the spell.
Silence - What do you need to say to cast the spell? Sorry can't hear you.
Grappled - You cannot move your arms to do the gestures or grab the component needed.

Alternatively (or in addition) he might show some mildly OCD behaviour. Patterns in his life that can be predicted and exploited. I've seen this work with someone who always ate in the same restaurant, at the same table, on a certain day - but you could play to hedonism and make it so that a large number of his spell slots are always taken by certain spells, making him much less formidable if he is unaware of a threat.

Thank you both for contributing! These will actually be very useful. I really wanted to know more about ways to beat casters. Particularly odd-mannered ones might have just given more ideas for the takedown.

As for justifying the mindset of this person, that can be a chew toy for a hydra. Fantasy is filled with so many exploitable plot devices, an incongruency becomes a tantalizing mystery. The explaination can range anywhere from magic omnipotence wasn't as exciting as one may think, to a yet more powerful creature dominating a new pawn.

Either way, I would like to see focus on ways to beat someone who might truly have it all. If you can tie it into the given personality, good. Just don't go all Gollum over it.