Metal Gear Solid 2: Sons of Liberty HD Edition Tips

Stop a guard from calling for help
Find a guard that's all alone, and shoot him with the M9 once to knock him out, then shoot at his radio until it sparks. Then, depending on the chapter you're in, wait in a corner somewhere until he wakes up on his own or wake him yourself with the coolant spray. Step in front of him so that he sees you. When he tries to call backup, he will discover that it doesn't work. That leaves him to try to fight you all by his lonesome instead of having a bunch of armor-wearing soldiers alongside him.