Okay here is what you do. First of all it depends when you do this. I done this after selecting my party on the GBA version. I had Cecil, Rosa, Rydia, Kain and Porom. Make sure Rosa and Porom have reflect. start out by casting Reflect on Rosa and Porom. Then Get Kain to jump. If you can, cast reflect on Cecil and Rydia but you might not have enough time. Mega Flare will be reflected back at Bahamut. If anyone died then revive them and cast reflect on them. Make sure to keep the reflect up. Get Cecil and kain to just attack. Have porom and Rosa revivie anyone or heal using items. Have rydia summon Leviathan. This is the strategy I used.