Penance is definently harder to beat than other people.
Arms do Mighty Guard which is nasty because it heals Penance and Mega-graviton which does near 65% of your maximum Hp as damage. The body uses Judgement day and obliteration two nasty attacks.

against Nemesis though you only verse one creature. He has slightly smaller stats than Penance. His moves aren't as damaging so he is easier.