You don't really need to change characters during battles, all you needed were Tidus, Wakka and Rikku. But I did because for Lulu and Yuna for gaining AP. Valefor is the easiest of the Aeons to defeat. Enjoy~! =)
Boss: Dark Aeon: Dark Valefor (Overkill) HP: 800,000 (99,999) AP: 10,000 (15,000) Weapon & Armor Drops: Break Damage Limit, Break HP Limit, Ribbon, Auto-Regen.
If you missed the chance to get Auron's Jecht Sphere on Besaid Island; defeat Valefor.