I found a really easy mission to do this in. In "Lin Cong I Presume", the one where you find that ceremonial knife, you'll eventually reach an area with a few ziplines. You'll even see one enemy go down the really long one. He was the one I got the achievement off of. After he goes down the zipline, you must also, and if you're lucky like I was, he'll still be hanging around at the end of the zipline. Just remember to press A to drop down on him before you automatically get off by reaching the end. Shortly after, I got another one of these kills, so if you miss the first guy, you still have another chance.