Bonemass patern is simple:

- he summons minions
- he punches you
- he makes his special poison cloud

The poison cloud is what you want to avoid the most, even with poison resist mead.
The punch you can parry very easily leaving you some time to do big dmg bonus.
His minions should be dealt with first but as far from the boss as possible.

Rinse and repeat, easiest boss fight in this game but the most fun too imo.

Needless to say to bring a blunt weapon and poison resist mead here. Have fun with your new Bonemass insanity power after that !!!