When I go up the Fanatics' Tower, I have Wall Rings on all my characters...makes things a lot easier, and only non-focused or non-specific spells can get to you (Merton, Ultima, Meteor, Whirlwind, Quake). And then, instead of wondering what enemies are automatically Wall-ed, you can just reflect spells off your own guys. Heal between battles, or with items, and you're set.

I agree with Cyraniel--there shouldn't be any bosses out there for the sole purpose of a difficult fight. Save, perhaps, the Monster Arena in FFX, that wasn't a bad idea. The final boss should be the most difficult, that would seem obvious to some.