Invincibility (self):
Select the "Enter Cheats" option at the main menu, then press B(2), X, A. Your first soldier will be invincible, but the remaining members of your platoon can still be harmed.

Invincibility (team):
Select the "Enter Cheats" option at the main menu, then press B(2), X, B.

Refill ammunition:
Select the "Enter Cheats" option at the main menu, then press B(2), X(2).

Autowin:
Select the "Enter Cheats" option at the main menu, then press B(2), X, Y.