If you're willing to sacrifice a significant part of your horde, you can keep most of your horde near the jail cell to the right, and then have have a mid-sized, but just large enough to get to the gate group of Zombies move towards the policemen. As soon as the group is close enough to the gate to destroy it, quickly use the detonate powerup on a random Zombie from that group near the gate. This will knock a majority of the policemen unconscious for a brief moment. Now you will have to make sure to move fast or this will not work. As soon as you detonate a Zombie near the gate, press Ctrl-A on your keyboard to select the rest of your horde. Now click on the unconscious policemen. While your zombies are rushing in towards the policemen, you can have a small group of your Zombies break away