This is done by using
to throw a rock. When you find a group on enemies preferable in any outpost you haven't taken yet, sneak up on them while remaining undetected and used
(down on the d-pad) to throw a rock and distract them. Do this to 15 different enemies and the achievement will pop.
SIDE NOTE: sometimes it does not register when you distract an enemy and I found it not to work on enemies that have detected me and then I waited for them to "lose sight" of me.
DOUBLE SIDE NOTE: If you are having problems with it registering, try spamming
over and over until you either hear a "huh" / "what was that" or see the enemy start to look around then check to see if it counted. I would sit in one spot near a group of guards and just spam
for a solid 30 seconds and then check the achievement tracker and it always seemed to work.