HideoK on 20 November 2010
Finally got the crossbow trophy and platinum! 
I have 2 extra tips to get this trophy:
1) Its a good idea to get all players to transport the the projection booth where the pack a punch is. This gets all of the zombies to congregate inside the theater room.
2) Make sure to have the 2nd player face you when you shoot the arrow. Then he can turn and face the zombies to blow it up. So shoot the crossbow into his chest.
With the 2nd tip I was shooting a friend in the back and his body must have been shielding the explosion because nobody in front of him blew up, only the zombies behind. Took me a few times shooting in the back and not getting 6 kills to realize this.
