A great level to get this on in campaign is Gulag. Once you get to the armory pick up a pair of pistols and wait for the door to jam. Enemies will congregate at the end of the hall. Just start picking them off from inside the armory. Once the door opens Soap will draw fire with a riot shield leaving almost no one shooting at you and giving you a fantastic opportunity to just pick off the enemies with your pistol with little to no return fire.