Infinite Invisibility and Ammo.
Okay, this cheat takes a little while to get to, but you can do it on any skill level, coop only. The Assault on the Control Room is the setting. First off, get to the part of the level where you start to go underground after the first large battle on the surface. There will be an active camoflauge to the right of the downslope before your descent. Leave it there and go underground. kill all enemies and cross the bridge like you were leaving. Out the next door in the big hall there is a loading/checkpoint that will drag the character who is lagging behind through. This is the glitch spot. Do not go through yet. One person needs to stand by this spot while another person goes back to the active camo. Make sure to pick up two good weapons on the way. My prefs are a Rocket and sniper. Equip the weapon you want to start with infinate ammo and stand next to the camo. The player back in the hall should get ready to pull you through. Pick up the camo, and have the other player pull you through the load point and then turn and pull you back through going the opposite direction. You should be pulled through twice. If don correctly, the second time through, you will see your head glitch a little. Now you are permanantly invisible and have infinate ammo. Be careful not to get on a vehicle, or the glitch will no longer work. Also, you can only switch weapons to secondary once. After switching back to your primary, you no longer have infinate ammo, so make sure you get as much use out of your primary weapon as you want before switching to secondary.
