Penance's main body has 12000000 Hit Points.
Every arm has 500000 Hit Points.

The Attacks of the arms are Curse of the Pharao, Mega Graviton, Mighty Guard and normal moves. While Ultima Shadow is right about negating the positive stats you need to be careful about the other arm as well (the one protects with the Mighty Guard the other one uses those Mega Graviton spells against you). You can break your HP Limit but it is not necessary (still I do understand it if you want to, it is just cool). Nevertheless Auto Haste, Auto-Protect, Auto-Phoenix, Ribbon, et cetera. Such abilities are pretty much obligatory. Don't forget that the Curse status ailment is NOT prevented from with Ribbon.

Penance has a very "slow" phase the first 3000000 Hit Points. It attacks in a way you don't need to worry about and the arms themselves are more of a problem. As Ultima said, be sure to have high Luck stat. Luck replaces many things and stats. You don't need a high evasion stat with it, you get many criticals, and, and, and. I hope you have the Beginner Sphere Grid because it has more slots than the Advanced one. Be sure to use Rikku's Overdrive alchemy as often as possible to mix Auto-Life, and all the other good stats into one move and save time. It would also be not to bad to have a Magic Point consumption of either "0" or "1" and better be sure that you prepare the arms to die after a while with a few moves so you don't need to worry about their attacks for a short time and because in phase 2 Penance needs both for its ultimate attack. Penance is more aggressive in phase 2 and it restores the arms in general but that is something which is actually not really the problem. This battle is a battle of patience after you have found a good strategy. I could give you more tips but for now its enough.

And: You can flee during the battle. The normal treasure for every time you have destroyed an arm is a Dark Matter.