More of an amusing discovery than a cheat,
Arondil's experiments in cave called Yngvild (or at least the last one of them) are susceptible to Frenzy. I found this out accidentally - cast it on the necromancer himself (only to see that he is too powerful even for my perks). Then I started looking for a place to hide, and was surprised by combat sounds. Arondil's experimental ghost got frenzied, and somehow killed him. Perhaps he was casting frost spells and it was immune.