Is the Rakh'likh quest line broken?

I can handle most of it solo just fine until you get to the part where you have to kill the three people: Allistarj , Grol, and Lady Severine. They were all immune to the magic I could cast: fire/shadow, so I couldn’t do anything. I got a group and we did it, then we did the final quest: You Are Rakh’likh, Demon. The main guy is also immune to all magic except a poison wand that our priest said worked against him. Is he and his minions supposed to be immune to magic? Doesn’t seem fair that if you pick a caster class you do nothing for the last parts of this quest because you can’t do anything.