When you start the game get on the bus and take it to the diner.

Build the zombie shield and go back through the mist (knifing the Denizens so you don't die).

Then when you get to the underpass keep going until the mist clears and you're at the area with the Colt M16A1.

From there go left of the gun and walk all the way into the corner until you can't walk anymore.

Then turn around, pull out the zombie shield and place it on the ground and if you did it right then the zombies pile up in front of you at the cars!

But be careful because if you move too much the zombies will come up and destroy your shield.

Your ammo will also have to be replenished appropriately.