Start by immediately opening the gate and run into the hub. Get what zombies you can in the stores, then get all the loose zombies in a bunch. When they are all grouped together go into beast mode and melee them. This does not kill them but causes them to respawn giving you another opportunity to kill them behind the barricades. Run back and forth between the starting area and the hub and eventually, given enough time, they will respawn. Using this method I got the achievement in round two.