Please read the other solution if you're unclear of something. I unlocked this on the Definitive Edition of the game, however this should work the same way for the 360. NOTE:
If the enemies were killed on this tower at any point in the game, you cannot do this method1)
Fast travel to Helicopter Hill2)
Connect the rope to the tower shown in the video. 3)
Now use the zipline and go up to the tower with 2 enemies.4)
Immediately ride the zipline down.5)
Hide behind the small structure and wait for them to come down the zipline. (They won't come if you are visible. You must be hidden from them.)6)
Equip your bow and draw back on it all the way. Press
to slow down time, if you are having trouble. Only go for headshots! This is very important.
This achievement is really glitchy and it took me well over 50 kills to get. Once I realized nothing else was working, I used my bow, pulled back all the way on it, used
and went for headshots. Sure enough, after my 4th headshot, the achievement unlocked. Getting 10 headshots still might not unlock the achievement. Instead, go for 20 headshots before completely giving up on this method.
If the bow doesn't work, I've heard of people using a shotgun instead. This is more trickier to do though, as you need to time your shot almost perfectly.
Also, make sure that the enemies die while on the zipline. They will die, and it will look like they are floating in the air. This is also how you do the multiplayer achievement for killing an enemy on a zipline. It must be done while they are on the zipline, meaning headshot.
If neither of these methods work, try shooting the enemy once with a pistol then go for a headshot. Good luck :) Video credit by me.