Also, Oni are hard to kill:
You'll see the basics from this pic, but there are various little things that I slightly affect various stats in here. He survived his first battle, but as the second one was against Lizards, they did him in. He still got Heroic Quickness from his first win, and some experience, and then got cursed and netted some afflictions in the lizard fight.
These are the biggest Oni, but even the smaller ones have second forms.