This is a lot easier than it sounds; pick one of the earn stealth challenges. You will want to take your time; don't rush this as you may get caught. Drop down and hold the
to sneak up behind enemies. Press
to perform a silent takedown; once you have killed the enemy you are after, make a quick escape to one of the gargoyles or another section of the map.
Using other weapons or takedown tactics will cause you to miss out on this achievement, so make sure you don't use the Batarang, Batclaw, or glide takedowns.