It's a good idea to do this right away. Start the game and head to the Quarantine Camp on the map. Check the ambulance and after the cutscene, you will be given a cart. Hold to push it forward. You can now run around town knocking over and killing all of the zombies. This achievement must be done in one sitting, as restarting a game with your stats will reset the zombie kill counter to 0. Once you get your 1000, you should be leveled up to or almost to level 3. Now, hit , go to quit, and restart using your stats. You now have the achievement out of the way and have a good head start on your level. Getting all 1000 should only take around 20-30 minutes.