Stealth
On the 4th level, after you drive up to the mines and get out, be Foley, and go up the little ledge. Command your men to stay back. Then, just take out all of the guys you can see. Foley has a silenced sniper rifle on this level so they won't notice you unless they see you. Try to make all of your shots head shots or upper torso shots. You'll know when you've killed every guy in that area because it will say "Objective Completed."
Cheat Mode
At the main menu, press L, L, R, R, X, X, Y, Y, B, B to unlock the cheat menu in Options.