Shapeshifter in Troll Witch Domain

So I'm trying to do the Kill The Troll Witch quest. I got into the Troll Witches domain and I can only get so far before being teleported to another room with a "shapeshifter" and nothing seems to hurt him. I assume there's a weapon I'm supposed to have but I'm not sure what it is. I feel like one of the characters told me but I can't remember. I have the Roar of Thor but that does nothing either. What am I supposed to do?

You need a sword called the Shapeshifter's Curse.