It's Issaru and Penace is fightable after you beat all the dark aeons by the way. To fully power up yuna's ultimate weapon you must beat Belgimines aeons at remien temple (Get a chocobo and head towards Macalania in the calm lands and find a feather to chocobo jump over to the path that goes there) including Anima and the Magus Sisters

PS Any more questions PM me