At the main menu, highlight the "Options" selection, then hold L + R + White + Black. Alternately, highlight the "Credits" option at the main menu, then press White, Black, X, B.
While playing the game, press L + R + A + Down. Alternately, press White + R + A + Down while playing the game.
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.
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.