When using the sniper mode of the SC-20K, you can hold L1 to make Sam hold his breath and aim more accurately. You can use this to clear out rooms or open areas from afar without letting anyone spot you. The safest way is to wait for a guard to stop in his tracks. Then, hold L1, set the target on his head and take the shot. This is extremely helpful in the first few levels from when you pickup the SC-20K. However, when you get to the Abbatoir mission, you must be extremely careful as the guards can see you from a lot further, can see you in the shadows, and also have more powerful weapons. You must exercise extreme caution. From Abbatoir onwards, if there are multiple guards in the one room, your best choice of action is to use a smoke grenade, if they are bunched up. If not. then do not go in commando-style because the guards from here on out are much more powerful than in previous levels.