Jiggig Skogre slaying

One of the slayer tasks I undertook this week had to do with skeletons. I donned my mage outfit, collected runes to crumble the undead and took off to the Ogre Jiggig. Whatever I tried I seemed unable to get the spells to hit. I don't understand: Skogres are clearly undead, resurrecting right before your eyes? There must be strange powers at work here, the Salve amulet also did not help a bit while fighting them. It's back to the old composite bow and brutal arrows for now, but I keep having a nagging feeling that something is wrong here.