The first time I killed him, I built a small platform three ladders high on top of the huge skull from where Bonemass is summoned. I summoned him, quickly climbed up to my little platform and casually shot him with many arrows until he was dead.
Did I die? Nope, not once. At that height I was well out of range of any of his attacks and the mobs he spawned couldn't reach me either. It was like shooting fish in a barrel lol.
Some, might call that cheesing... some might call it smart planning ;)