The strategy guide states he's weak to ice, but the ice wand does about 20 damage to him, implying that he has no actual weakness to ice, and in fact, a significant resistance.
Anything else with a weakness to ice takes a lot more damage, and that was from the main character at level 13.
Being almost 18, he shouldn't take nearly zero damage. He at least is annoyed by Gravity and others.
I didn't try fire, because that would be silly. :P
Just thought I'd point out a bug I found.