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I think this guy would do better as an assistant to the BBEG than as the BBEG himself. The issue is action economy - sure, the boss can use up all his actions to shut down 1 or even 2 of the party's casters, but then the rest of the party will just kill off his minions and then kill him. You're better off making Mr. Dispel the second-in-command, and let him lock down the casters (even if not quite as effectively as the BBEG would - but that's fine, you don't want them to totally suck). This forces the melee types to choose between killing the boss without magic support, or targeting the anticaster and thus maybe letting the BBEG run up to and murder the casters.
Note that the above is also a common thematic scenario: kings almost always have wizards around to protect them from magic/scry on their enemies, and sorcerous cultists have a habit of summoning demons more powerful than themselves. Variants on those two scenarios are limitless.
Also, be very careful with this type of thing - if you've ever played Magic, you should know why blue counterspell decks are reviled by most casual players. Wizards did a study - apparently, it's more fun to play a creature and have it blown up than have the casting countered in the first place, even if the end outcome is the same.