The one-shot sneak attack is one of the greatest weapons in gaming, allowing you to sneak up on unsuspecting foes and stick a very sharp dagger in their ear. It’s a classic, and it is something that will be available to you in Ghost of Tsushima. You won’t be able to do it from the start of the game, as Jin sees himself as an honorable Samurai who would not kill a man if he could not look him in the eye.
As you play through the story, Jin will realize that if he really wants to save his people, he will need to use tactics that may not always be honorable. You will eventually come to a Tale that requires Jin to sneakily kill guards in order to save the lives of the prisoners they hold, and from that point on you will be able to sneak attack and back stab enemies.
The most important thing about sneak attacks is moving slowly. You need to make as little noise as possible to sneak up on a guard, and when you are close to their back you can hit the Square button to kill them. You need to do this outside of the line of sight of other guards, or they will see you and raise the alarm.
You can also use your sharpened sense to find guards that are out of your own line of sight, so you can avoid initiating an attack just as some is about to round a corner and see you. To do this, push the touch pad. When you are in this mode, nearby enemies will take on a faint red outline, allowing you to see their locations, but you will not be able to move very fast. If someone is about to turn a corner and see you, press the touch pad again to deactivate it and quickly get to cover.