Leon is not that hard to beat, if you keep your people free of bleeding. Mithril heavy armor helps. Even steel + bone swords are okay. Provoke him if possible, poison and bleed him. In the end he maybe kills someone (1 or 2) and will die from fragile state. I am more worried about Nixx. I can't beat it without fully reorganizing party. And when you are that deep into game, you don't do it. You just quit.