Bonemass telegraphs his attacks, and has a regular interval between them. This makes him very predictable to fight if you take your time and don't panic.

To defeat him, I brought an Iron Mace and Banded Shield, as well as an Iron Sledge. I used the shield and mace to fight him head on; attacking when there was an opening and parrying his attacks for a counter. I used the Iron Sledge to destroy his skeletal minions using its AoE attack, which often took care of them in one blow.

Throughout it all, I maintained a Poison Resistance mead.

If you switch between fighting Bonemass and AoEing his skeletons, it's pretty straightforward. When he does his AoE attack is typically a good time to AoE the skeletons.