Penance is the ultimate superboss of the game and requires you to grind out a lot of stats for your characters. You will want to max out Strength, Defense, Magic Defense, and possibly Accuracy and Agility before fighting it. You will need to defeat all of the Dark Aeons, then you will find a listing for it on your airship’s navigationg. For more information about the preparations for the battle against Penance, as well as the battle itself, click this link here (Penance).