Using my 59 Druid, with Blue Diamond gear and buffs, I got my Cold Resist to 204. I resisted the Ice Comet spell. Good thing since I only have 1700hp! =) This was the only spell he tried to cast, just the one time. I tried to stay outta range once I had him rooted. He dropped the mask the first and only time I killed him.
IF you have an extended range item you can root him and back off out of his range and keep him parked and dot him to death easy as pie.
Also if your not racing to be the first to tag him, he will agro many of the other mobs and in so doing lose a bit of hp and more importantly most if not all of his mana (saw him slay a few froggies before i attacked him).