The aim of this achievement/trophy is to kill 6 zombies with a 'pack-a-punched' crossbow that you have shot into a teammate OR be the guy who gets shot. This can be tricky as the whole achievement/trophy relies on luck as the earlier you get your crossbow and get it 'pack-a-punched', the more likely you will do it. If you are still trying to get this at round 13+, zombies caught in the explosion are likely to become crawlers and resulting in you not getting full 6 kills.
Try to get to the box as quickly as possible and if luck is on your side you will hopefully get the crossbow on your first few tries of the box. If you trust yourself, dont bother getting any perks as the sooner you get the crossbow 'pack-a-punched', the sooner you can carry on playing normally but it is recommended that you're teammate has juggernog. When you earn 5000 points just leave a few zombies and teleport to the PaP room to upgrade the crossbow. Now thats out the way, if you have not already, survive the first dog round otherwise keep the crawlers alive until you can relink the teleporter. When it is linked, kill the crawlers and stay close to the teleporter.
When the next round starts DO NOT kill any zombies and get inside the teleporter. Activate when zombies are close but not close enough to be killed by the electricity. Again leave the zombies so they form a big crowd in the theatre and wait until you arrive back at the spawn room. The second you get back, the zombies will already be coming from the stage up the hallway so you must be quick. Have your teammate stand still so you can shoot him with the crossbow. If you are too close to him you can't shoot him so you will have to back off a few steps. When he is shot, the Crossbow bolt now acts as a miniature monkey bomb so the zombies will surround him but wont attack. Upgraded, the bolts will have a longer fuse so your teammate will have enough time to run into the approaching horde. When the bolt explodes this should kill all of the surrounding zombies bagging you and your teammate the achievement/trophy. If you didnt get it just leave a crawler and have the teleporter re-linked before the next round and repeat. Template:CoD7 Achievements