At the start from whatever position head to the barn.

On your way you can grab the ammo box beside one of the blown up vehicles.

One man sets up on the left side of the barn another behind the sandbag.

One can rotate between snipping on the second story and helping out below by crouching down behind one of the pillars.

Once the Boomers come out and you've killed a few take their shields and plant them in front of you for protection.

The bigger guys can't get through but the smaller guys will knock them down.