Mace and Armour... (Poison Resistance Potions)
Every time new ads spawn pick them off first and then go back to attack Bonemass..
Dodge his heavy attacks.. You can also block these attacks. (Round shield)
With Poison Resistance you don't need to dodge his AOE Poison attack, but it's better if you can.

I don't suggest using arrows of any kind, they deal low damage.

The fight can take up to 20min depending on your gear and what level your skills are.
He's fairy easy to beat.