Penance is the most powerful Boss Monster in FF10 even stronger than Nemesis at the Monster Arena. Penance can only be fought once you have defeated all of the Dark Aeons then simply select Penance as the Airships next flight destination. After doing further research on FF games i've found out that Penance is not in the USA version of FF10.
Penance has 12,000,000HP, all stats at the maximum 255 other than defense and magic defense which are about 230 and his arms have 500,000HP each with stats at least 200.