On our current world, we decided to try a different tactic for fighting Bonemass. In retrospect, the bonfires were mostly pointless, but the platform worked out pretty well.
A couple minutes into the fight, I discovered that I could reach Bonemass with my mace from the platform, by taking out the half-walls. While a bit dangerous, this greatly sped things up.
Did that work well?
Took some adjustment, but very. The structure was high enough that Bonemass wasn't damaging it or us directly. With poison resist mead, the poison he was putting out wasn't a threat. The skeletons and blobs he tossed at us were mostly a non-issue, though a couple needed to be smashed.
We started off just shooting him in the face with arrows, but this was slow going. I then removed the half walls around the perimeter and tried hitting him in the face using the mace alternate attack (middle mouse button). That worked spectacularly. I could swing, hit him in the face, back up a touch and repeat.
Took about 10 minutes to kill him (I recorded it to get an idea). But much of that was faffing about with arrows. Once I figured out the face smash, things sped up considerably.