Before fighting Penance, defeat all Dark Aeons. Armors will be Break HP Limit, Auto-Haste, Auto-Ribbon & Auto-Phoenix/Auto-Protect. HP must be 45-75k, MP at 999, Core stats including luck at 255, sacrifice some Aeons, sacrifice all gil (about 10 million) to let Yojimbo use Zanmato if you are lucky enough, though. You should beat him for about 30-45 mins.