Mark of the Ninja: Remastered

You cannot get this until you have the upgraded cardboard box, which becomes available to you around Chapter 10 or 11. The first cardboard box only allows you to sneak past enemies, while the upgrade gives you the ability to assassinate them when they pass by you. If it's your first playthrough, go back to Chapter 1 after you complete the story and get the achievement there as it will be super easy. The last few chapters have way too many advanced enemies to bother with it.