Better way of doing this level is to pick up the rifle (not the bolt action, the one that has a 20 bullet clip) near the start of the level. This is far and away the best gun for doing headshots on the zombies. Do not lose this gun.
When you get into the temple, you should have full ammo. Stay where the roof collapses in, and pick off zombies as they march towards you. When they get close resort to meleeing with the rifle (B button) as this is a very effective way of getting them down.
Once they stop coming move closer to the middle island, this will spawn another wave. Run back to the start again, and finish them off as before. Keep doing this (inching forward, then run back) until you get "Checkpoint reached". Now it's time for the final "boss" (who funny enough you've already fought before 2 times).
On the island in the middle is the M60 and some ammo. Pick up that gun (dropping your rifle -- it's useless against the boss), and ammo. You should be able to go into the final boss fight with an M60 and full ammo (100/90). Once inside just unload on him, it shouldn't take even a full belt of ammo to bring him down.