Normally I'd say 'go devour stuff' but if you dont' have eden yet. .. xD

ALTHOUGH, you can teach it to one of your GFs if you have a hungry cookpot. You can get those by angelo searching (hit a turtapod enough to put it in its shell, then cast confuse with rinoa in your party and wait) You might be able to refine a shaman stone, but I'm not sure how to get those outside of refining them, maybe you get dark matter and use tool refine? Dark matters can be refined from curse spikes-- collect a lot of tri-point and tri-face cards to refine them, or fight the actual tri-face monsters on the island closest to heaven.

I'd say that if you have a game save before you levelled to 100, use that.

There is NO USE in levelling before you get bahamut. It's just. . . stupid, really

that being said, you might try to figure out a way to get auto-shell, since it seems to me that mega flare would be magic based, I think 100 marlboro tentacles makes a glow curtain, which is auto-shell? or it could be any other number of things-_-

best choice is finding a non-level 100 save state, honestly.
or asking someone that knows more about bahamut