Level select
At the main menu, highlight the "Options" selection, then hold L1 + R1 + L2 + R2. Alternately, highlight the "Credits" option at the main menu, then press L2, R2, Square, Circle.

Invincibility
While playing the game, press L1 + R1 + X + Down. Alternately, press L2 + R1 + X + Down while playing the game.

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

Sniper rifle
Always use a snipers rifle. This seems to work the best, because you can stay low and pick your targets. If it gets too hot, send your team. If you tell them to spread out, it works better than an attack.