It's been a long time since i went toe to toe with Penance.

Listen to Karifean, you must concentrate all your attacks on the arms before attacking penance. Why? Because in his second form, if one arm gets an attack while the other arm is alive, you will be greeted by his Ultimate attack. That's 99,999HP damage that dispels auto-life as it hits, so it's no survival.

You want to abuse quick hit for all it's worth, and quick pockets for elixers in the second form. Elixers are essential because in his second form, whoever he targets wont just take a ton of damage, but lose all their MP too, which means no quick hits which = you dead in a few minutes.

From what i remember, hitting for max damage (99,999HP) means you kill each arm with 6 hits. You get a few turns to do damage and make sure you're healthy before the first arm regenerates. On the 6th hit and as you kill that arm, the other regenerates and you quick hit that 6 times. A few turns to attack and it's rinse rinse repeat.