I'm not sitting in front of a book, so some of my terms are probably off, but this is something I've played around with a little. I'd break the various classes (prot. against dark arts, potions, etc...) into seperate skills. Then have the the spells be powers with the skill as the success roll. Maybe a power framework or something would be better, but I'm not super familiar with all the Hero rules yet. Harry seems to be particularly good with attack spells and flying while Hermione has a good overall skillset. I assume it was easier for Harry to do Crucio in the last book because he'd had so much practice with attack spells in the previous book. I don't know if Crucio being difficult gives a penalty on spell success checks or needs its own check....