To be in 'skeleton form', a player needs to die in a multiplayer game. Once they die, a grave appears where they fell, and the player continues on as a skeleton until revived and some time is shaved off the timer. The players who are skeletons can die, but come back again at the spot where their graves rest and shave off more time. If all players become skeletons, it's Game Over.
So, to do this, you'd need 6 players, one of which would stay alive. Easiest way to do this is the first boss, Gergoth. Just weaken it till it breaks the ground and you all fall to the first level of the tower. Then, 5 of the players kill themselves by running into it, getting hit by attacks, etc. After they're all dead, just procede to KO the enemy. Easy-peasy, and all involved get the achievement.