[Endoperez]

It's a pity there isn't Stone Warriors at Alteration 4. Iron Warriors at Alteration 5 makes the targets vulnerable to lightning, but Stone Skin's cold vulnerability would be negated by Ao-Onis resistance.

Still, alteration seems like the way to go for the Oni. I think Immolation could be a very interesting spell for them.

Kappa could be useful in late game. They have protection 15 and an armor-piercing attack. I'm not sure how much fatique they get from fighting on dry land, though.