Honestly, I accidentally cheesed this one by completely flattening every bit of ground around the spawn area. I was trying to keep leeches out, but it had the side effect of not allowing any adds to spawn in. I've heard that an AOE weapon like stagbreaker or the sledge can take care of them though.

Use blunt on Bonemass. Run when he leans back before the poison breath. Attack right after he tries to melee you. Lead him in circles around the skull. Don't ever forget the poison resist.