When he smashes the wall, and breaks the gate go through the gate. At the end you should see a huge door. Go to the door, and look the right. You should see a chest on the crates. Open it and there should be a machine gun. Grab it, and go to the Axe Zombie. Remember, go close and your a gone if it's not online/co-op. Aim at the boss's head and shoot it. He should kneel down, go up to him and use your martial arts option. Repeat this until the boss is dead, but if you run out of machine gun ammo use your handgun to shoot a few zombies.