This is not the easiest achievement in the game and will require you to probably have a few skilled players in the game.
Firstly, this achievement requires at least two people playing in Zombie mode on map Kino der Toten. Next, you or the other player needs to obtain a crossbow, which can only be found in the Mystery Box which is randomly place on the map. You can either open up the areas randomly, or you can head towards the auditorium to turn the power ON. Turning the power ON allows you to view various blackboards that have a chalk outline of the map area with light bulbs attached. The light bulb which is lighted is the location of the Mystery Box on the map. Head towards there, and use the Mystery Box, which costs 950 points. The weapon you get from the Box is random, so you may have to try several times. After you have obtained the crossbow, you need to head to the 'Pack-a-Punch' machine, which can only be accessed by turning the power ON. The 'Pack-a-Punch' machine costs 5000 points. In 'Kino der Toten', the machine is located in the projector room. In order to get into the projector room, you will need to unlock the doors to the auditorium, and then step on the teleporter pad to activate the link by holding X. Then you need to run back to the main room foyer where you started in, and initiate the link there. Finally, you will need to head back to the auditorium back to the teleporter to get to the projector room. You only have a limited time in this room, so don't sit idle. Make sure you have your crossbow weapon as your main weapon, and make a purchase from the 'Pack-a-Punch' machine.
After you have the 'Pack-a-Punch' Crossbow, the other player will need to be shot at with the weapon and the resulting explosion has to kill at least 6 zombies. The best way is to have the person with the crossbow shoot the other player, and have that player run into a group of 6 or more zombies.
All credit for this solution goes to XCVii 007 at http://www.xbox360achievements.org