The boss at the top of Kefka's Tower in FF6 would have gotten my vote a couple of years back. Only being able to use magic was a pain...and he kept changing his barrier at random. And then he nukes you with Ultima when he dies which has a pretty good shot of killing your entire party. But one day, genius struck.
I went to town and gave each of my dudes a wall ring, providing perma-reflect. The guy just shoots himself to crap and doesn't bother with the barrier change. Oh, and interesting fact, Life 3 goes through reflect so Ultima was nothing.