In the game buy a guard tower then go to sell when it starts to shut down hit "s" many minigunners will pop out and the tower will stay very useful(only works with guard towers).
Stealth units
For this cheat you must be NOD,and be in a multiplayer practise (i have not tryed with online games or LAN)and get about 500 $, in the Character Buy Screen, become a Black Hand (the most right one because its the cheapest) and kill one person, but dont be hurt. Then you must go to the Character Buy Screen again and become any NOD unit you like. If you did it right the person you are now is stealth just like a Black Hand unit.