Im guessing that the person who is getting oneshotted, is a melee? If not, get him far away from the boss. If he IS a melee, tell whoever is targeted by bonetrasher to stand the feck still and take it like a man! We've seen this several times, people running around like scared little kittens because they think they can outrun this boss. Truth is, you cant. Man up, eat that ****, dont kill your team. Follow this recipee, and you're golden!
If this wasnt it, and the melee was standing RIGHT behind him (not to any of the sides, but right behind him),then it's a bug, and should be reported with as many details as possible :-)