To obtain this achievement you must backhandedly kill 1000 Zombies. What I did to do this and obtained in well within 30 minutes is as follows:
1) Be a decent level, the higher the level the easier. You should at least have the
2) Stock up on Untouchable mixed drinks. You can head over to Paradise Plaza roastmaster and mix the 3 pies with 3 orange juices in the blender to get 3 untouchables.
3) Grab the mix drinks books or food increase books. The first book store to the left when entering entrance plaza from paradise plaza (progress through story to case 2-3) has a book that makes the effects last 2 times as long.
4) Go into the Maintenance tunnel where the maintenance key is then back up to the spawned 300-400 zombies.
5) You want to remove your weapon/food by pressing
and keep using this combo:
on a zombie then
. If you get hurt enough just drink an untouchable drink and you won't have to worry about being grabbed by the zombies. Each stomp will kill up to 7-8 zombies each time.