Daradus94,564 08 Dec 2010 09 Dec 2010
Just to add to what everyone else is saying, if you get past level 15 and they're not dying in 1 hit and you cant time an effective insta-kill just start doing laps of Kino Der Toten from the lobby to the stage, shooting zombies in the legs as you go. Kill the weird crawler ones as they're just a pain but when we did it it was round 18 and we had 8 crawlers at the end of the spawn, shoot, go prone and achievement unlocked!