Level select
Highlight the "Options" menu, then hold L1 + R1 + L2 + R2. Alternately, highlight the "Credits" option at the main menu, then press L2, R2, Square,Circle.
Missions and Weapons
Go to the main menu and highlight credits. Then enter L2, R2, SQUARE, CIRCLE.
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 him 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.
God Mode...