Go to the maintenance tunnels and make sure you have the Double Lariat skill (L3 + BLUE while Frank is motionless). Go into any big group of zombies and use this skill. The best place to do this is the area where Carlito's (usable) white truck is (it's the northwest corner of the tunnel area). Once you kill all the zombies in that area go into the butcher room and go back out and all the zombies will be back. It will take about then (real time) minutes to get the achievement. If you run low on life, you can always use the white van to head to an exit and come back later.