not sure if this works. it might.

1. get a black hole.(from gespar, gespar card, or diablos card) and teach quistis degenerater.

2. go to normal battle. pummel quistis till yellow hp

3. go to DEEP SEA REASERCH ISLAND, not battleship island

4. fight ruby, degenerate. repeat.

5. at bahamut fight, kill bahamut.