Health points aren't worth much when you're out of stamina. I use 1 health to 2 stamina in a build after early game.
I generally lean on Bonemass with the tower shield from above and pound him with the mace until I fall off, then run around the fort and repeat until he drops. Still have to avoid the poison gas, even with the root mask,. If I stay with him on the ground, HP gets low before I get very much done.

This time, I ended up standing on his shoulder or head, and couldn't hit him.