Have you found the trader yet? The iron sledge and iron mace work very well against bonemass. If I remember correctly, you also have to have a Draugr Elite trophy for the iron sledge, but it's a great weapon.

What I did the first time through bonemass was wall of a small area with tall earthen wall to provide a safe spot to heal if necessary, and then used the iron mace to pound bonemass, and the iron sledge to clear his minions. That, along with good food and poison + stamina mead will get the job done.