Humans will come down from the top, rushing through to the bottom of the level. Catch them simply by zombifying a few, then spreading your zombies over all the entrances. However, there is a line of 8 Men in Black standing at the top wall, shooting any zombies that pass the booths. Try to zombify them as soon as possible or keep your zombies away from them by sending them to the back wall. If you keep at least 400 zombies when the time is up, you will get the Stamp Collecting achievement; you can look there for more strategies.