This achievement is pretty easy, regardless of difficulty, and can easily be attained during Mission 1 or many other parts of the game. Ideally, you'll want to find an enemy that is somewhat alone and isolated. Take your time using cover and stealth to sneak up behind them, and then you will get prompted to stealth kill them using click. Do so while cloaked, and you will automatically cloak after getting the kill. Assuming that you weren't spotted, and enemies don't begin shooting at you, you were successful. An achievement tracker will show up during your journey to 20 kills. Just continue to use this method, and you'll easily get this achievement.