Bonecrunch is deadly to most players if he is let out of his throne room. Keep him inside by leaving the Wall of Fire up. The wall goes down if Captain Burnja is killed. Simply leave the wall up and have archers shoot Bonecrunch through the fire.
Bonecrunch has very high (600+) magic defense, but is not immune to either creature or life magic. An Imperil and Vulnerability combined with an Armor Rending bow make short work of him.
Mages with specialized magic skills can occasionally land debuffs on him. Wielding a Focusing Stone and having Brilliance cast on you by another player is the best way to boost your creature or life skill.