All bosses are easy if you are well prepared. For bone mass you just need poison resistance and an iron mace. He has slow attacks that can be dodged easily. WIth a decent amount of stamina and by running away everytime it gets low you can beat him without getting hit even once (except for the poison which is not worth evading with resistance potion).
If you play with a friend, one of you can take care of his minions and the other one can hit him meanwhile. Everytime the one hitting the boss runs low on stamina you switch and regen. Bring some stamina potions for emergencys.