This is a lot easier than it sounds, pick on 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 your 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.