Frostner will be an op weapon for bonemass but a fully upgraded iron mace will do the job well. It's an easy melee fight, pop poison resist, pop eikthyr buff. hit him with a three hit combo between each strike he does, run/jump/roll (your choice they should all work) directly away to dodge attack, rinse and repeat, recover when needed, even when he does the posion barf keep hitting, the posion resist makes it not even hurt you. Kill his adds when he throws them, easy yo, did it in level 2 iron gear was a cake walk. I think I used maybe 1 health potion.