There are two cheats which can really beef up your character. These are invincibility and instant full adrenaline.
After unlocking "enter cheat" mode by holding L + R and press Up, Y(2), B, Up, A. You then release L+R to enter one of the cheats:
Invincibility: B, Y, Right, A, Z, R
Adrenaline: X, L, B, Z, L, Y
Before you get to the Nemesis Bosses, fill up your adrenaline meter. This can be easily done by performing head shots. Once your meter is full, save it until you get to the Boss. Once you see him, press Up. You will now have limited immortality and unlimited ammunition. If you have a sniper rifle, you can shoot the Boss in the head a few times. If you have a sub-machine gun such as a Thompson or an STG-44, you can run up to the Nemesis and just shoot him. Another way to inflict serious damage is to use a rocket launcher. Note: You will not have unlimited ammunition for this; it is best to have a good back-up weapon. If you still have not killed the Boss and your meter is almost empty, take cover. Also, if there are enemies surrounding the Boss, which is true in most circumstances, eliminate them before you attack the Boss.
After the completion of a level, you will be rewarded one Medkit for each surviving team member for a maximum of three.
Finish a level and complete all the missions listed to receive one free revive.
There are mounted machine guns that you can use to shoot Nazis. This is not easy and is also risky because you are shooting from a fixed position and cannot duck or hide. To get around this, send your squad directly next to the machine gun. They will use it instead of you and effectively mow down any enemies that are near by.
Pause the game, then hold L + R and press X(2), Left, Y, X, A. The following codes may now be enabled:
Enable the "Cheat mode" code, then press A, Y, X, Right, Y, X.
Enable the "Cheat mode" code, then press Z, R, Z, L, Y(2).
Enable the "Cheat mode" code, then press X, Down, X, Up, Y, X.
Enable the "Cheat mode" code, then press Down, Z, L, Up, B. The person you have to hunt down in various missions will be killed.
Pickup OSS Document
Enable the "Cheat mode" code, then press Up, A, Z, R, Up, B.
Enable the "Cheat mode" code, then press Z, R, Up, X, B, A. Repeat this code to disable its effect.