Have you tried walk him into the fire and then stun him approach. Then you head over towards the water and heroic toss it when molten or worst case charge it and shield slam. Always found taunting it dangerous because of the ammount of healing going on they always get agro if you just taunt and if you keep taunting you hit diminishing returns and it goes immune.
Never panic if you misstime the stun and it doesn't go molten as you havent lost agro so just try it again.
** Remember Warcraft players fail in directions you never thought possible.