Don't bother with Aeons. Their special attacks are the only attacks that do damage, and they all have the world's WORST cooldown rates. (Pain makes you lose nearly 2.5 turns).

And yes, any Aeon that's not Ixion has crappy MDef until they all have 255. Even then, by that point, your magic should start damaging it effectively.

Jumbo Flan has alot of HP, and the best way to kill it is Doublecasting Flare off a reflected Character (The Flan has reflect auto-cast on it). Or doublecast Ultima (which ignores reflect). It is helpful if your characters have One MP cost, and Magic Booster. The problem is that magic (even Ultima) will not always do 99999, so don't expect an overkill anytime soon. Don't forget Hastega and Auto-Life.