Level select
Just highlight the "Options" menu, then hold L1 + R1 + L2 + R2.

Or simply highlight the "Credits" option when your at the main menu, then press L2, R2, Square, Circle.

God mode
Just press L1 + R1 + X + Down while playing.

Or you can press L2 + R1 + X + Down while playing.

Shooting difficult to hit far away enemies
Look at the enemy through the binoculars. Then center the enemy in your binocular sights, then switch back to your rifle. The rifle will now be trained on the enemy. Then fire a three round burst and you will take him out.

Sniper rifle
Use a snipers rifle. This will work the best, because you can remain low and pick your targets. If it becomes too hot, send your team. If you tell them to spread out, it works out better than an attack.