Game of Bones The achievement has been fixed now so if you have bone armor simply equip it (which you obtain it at lvl 19, and it is preset as
) use on the Skeleton King and this achievement will unlock.
If you have already been to the location of the Skeleton King you can just travel to Act 1 Royal Crypts.
If you are having problems unlocking this (pre-patch players) then refer to my other guide that has a delete save (able to back-up) walkthrough to unlock this achievement: