Believe it or not, there are aeons even more powerful than Bahamut. In the Calm Lands, circle past the right side of the bridge leading to Mt. Gagazet, and descend into the gorge below. To the left of the Save Sphere is the Cavern of the Stolen Fayth. Proceed through the cave until you encounter the spirit of a dead summoner. The summoner will call Yojimbo. This aeon has high physical defence but low magic defense, so hit him as often as possible with spells such as Firaga. Afterward, you can use the teleport pad there to travel to hidden chambers containing a Flexible Arm, and MP Sphere, and two X-Potions.

Next, travel to the Chamber of the Faith and speak to Yojimbo to negotiate a fee for his services. He will offer his lowest initial asking price if you choose the option "To defeat the most powerful of enemies". For your first bid, offer half of his asking price puls 1 Gil. So, if he asks for 250,000 Gil, offer 125,001. After his next offer, raise your bid by 1 Gil. By the time he proposes 205,000 Gil, he will generally accept offers below 200,000 Gil, though not much lower. If at any point you him at least triple his asking price, he will give you two Teleport Spheres in addition to his services.