Start by advancing forward, taking out anyone in your line of sight. Once you get to the intersection, you should be able to vault into a small room. Pick off everyone with your semi-auto until the truck comes through the gates down the road. Run to the truck to trigger a cutscene and checkpoint.
Immediately grab the Assault Rifle and ammo and hop on the mounted gun. Finish the sequence and take a right (facing the hill). Work your way till you have to defend the bridge.
Immediately grab the RPG and the ammo and load a rocket, and then pick up the AR again. Take them out once they stock running across the bridge. Near the end, when they group up, shoot the rockets. Sooner or later, the gates are going to be compromised and you'll need to haul ass to the church.
Immediately reload/resupply and take cover behind the barricade in the far back. Take out the enemies as they come in, and if they get close, melee with B and headshot. When the door behind you opens up, drop into the tunnels for another level down.