Invincibility:
During gameplay press L + R + A + Down or press White + R + A + Down.

Unlock Level Select:
Highlight the 'Options' selection at the Main menu and then hold L + R + White + Black or alternatively highlight the 'Credits' option at the Main menu and then press White, Black, X, B.

Shooting Difficult to Hit Far Away Enemies:
Locate the the enemy through the binoculars centering them in their sights. Now when you switch back to your rifle your rifle will be trained on the enemy. Firing a three round burst is normally enough to take them out.