Ignoring the policeman, have your zombies head to that location as quickly as possible, using your speedy zombie to get the Miasma powerup in the bottom right. Try not to lose too many zombies to the laser gates. You should have two Miasma powerups and a few humans captured (except on Hard, where you only have one Miasma.) Cross another set of lasers, trying not to lose either zombies or humans to the alien, and place a Miasma in front of the enemy zombies.