This is really odd... I never had problems with him myself. With any magic dispelling weapon, stoneskin is done. Dispel Magic will make it go away as well. I'm not sure if I died the first time I fought him or not... I was a kensai/thief though, dualed in ToB, so fairly high level kensai with excellent armour. I just kept pounding the crap out of him with my dragon stopping halfing... The Answer/Sword that does double damage on dragons dualed with Black Razor kept him from lasting long. I just dispelled his stoneskins and kept on hitting him.
I don't remember him using Protection from Magical Weapons much. lol, even if he does, just hit him with non-magical versions after you dispelled stoneskin.
I might have sent in some summoned critters with my halfling though, I can't remember. It was a while ago. lol