Upon retriving The Invincible as your second airship from the Anicent Ruins; you are thusforth able to obtain the eighth level of Summoning Magic, Bahamur. First jump on The Invincible and return to the floating continent. Enter from the east side of the floating continent and fly straight until you see two caves, surrounded by mountains. Enter from the east low mountain by using The Invincible's Overdrive, by pressing the A button. Enter the cave and you'll enter Bahamut's Lair. Explore the dungeon - there are basically chests everywhere; they really offer a lot of things new and not so new. When you reach the end of the cave, Bahamut will speak to your party and you'll exit into the second cave you saw on the world map and automatically engage into battle.

Your battle with Bahamut will be pretty simple - around level 39-41 should do the trick, or be the bare minimum. Bahamut will do the typical two strikes per turn that all bosses do, he'll do his basic attack or he will use his best move. Mega Flare, which will do probably above 1000HP of damage. Just keep up the damage - the White Magic spell: Aeroga and the Dragoon's Jump ability are extremely good against an aerial foe like Bahamut. He'll drop the level eight Summon Magic spell: Bahamur.