Since NO ONE on this site seems to acknowledge the below information on this site, I thought I would post it, after searching high and low for the information. This is useful for anyone starting the game now, like me, and just want to run through it quickly.
You can obtain all but 2 campaign achievements in the game using these cheats, as the cheats only work on easy, but do not disable achievements at all. Some might say this game is easy enough on easy without achievements, but again, this is for people that want to run through it with no care in the wind.
Side Note: These cheats cannot be activated on Mission 0 (I found this out the hard way, buy thinking they didn't work). Once you complete Mission 0, and begin Mission 1, the cheats all work fine, and I tested them as of May 01, 2012.
Side Side Note: You must enter in each of the cheats at the beginning of each level, and can enter the Thermal Energy one as needed. Enter each cheat from the pause screen (It is probably a good idea to collect all Easy Target Marks on this run-through also, since you can't die except falling)
- Infinite health
Pause the game and press Down(3), Up, Y, Up, Y, Up, Y, Up(3), Down, X, Down, X, Down, X, Left, Y, Right, X, Left, Y, Right, X, RB + LB. If you entered the code correctly, you will hear a sound.
- Infinite ammunition
Pause the game and press RT, RB, Y, X, Right, Down, Left, LB, LT, RT, RB, Y, X, Right, Down, Left, LB, LT, RT, LT, LB, RB, Y, Left, Down, X, RB + LB. If you entered the code correctly, you will hear a sound.
- 500 Thermal Energy points added
Pause the game and press Up(2), Down(2), Left, Right, Left, Right, X, Y, RB + LB. If you entered the code correctly, you will hear a sound.
These cheats are found on various sites by using google, but none of them tell you all the extra information I posted above...since I tried looking, and had to test it myself.