When I fought bonemass, this was just after the game launched, and I built an entire fort directly on top of his summoning skull. Did the whole fight from on top of it, and hardly took any damage.

Apparently they fixed that, and bonemass does some horrible thing that launches you into the air like being shot out of cannon.

But it still might be worth building some kind of raised platform. See if you can build something around the trunk of one of the big permanent (unchoppable) trees in the swamp. just don't build it directly on top of his summoning skull/alter.