To get this, you'll need to let an enemy spot you, then you have to hide without them seeing you, and finally kill them while they are still searching the area. Do this five times to earn the achievement.
Two really good spots to do this:
- The first side mission in Kuwaq Yaku, "Deal with the pillagers". When you encounter the enemies, let them see you, run around the corners and hide in a bush or against the wall. When they follow you, take them down.
- After leaving the Temple of Life, you'll come to an area with boats, and some enemies on a dock. Let them see you, then swim under the dock to get out of their sight, and do the takedown from the water.
In both cases it is a good idea to turn on the achievement tracker and just reload the checkpoint after the kills are registered. (not working on highest difficulty due no autosave present)
If you downvote please do me the courtesy and comment how can I make the guide better. Cheers.