I built a platform directly on top of the summoning skull, with a winding staircase to the top. I remember getting hit by 2 attacks during the whole fight, and neither of them was even from Bonemass. A blob jumped up and did the toxin explosion thing in midair after bouncing off part of the platform. Got clipped by it.

I just shot bonemass with arrows nonstop. Eazy Peazy fight.