Press L + R + A + Down during game play.

Highlight the "Options" menu, then hold L + R + Black + White

Code - Effect
During gameplay press L+R+A+Down (on d-pad) - God Mode
Highlight options and hold L+R+BLACK+WHITE - Open All Levels

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.

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