10 enemies shot off zip lines.
Enemies can go on zip lines, and will do so when rushing to attack you. Shoot them on their way to instantly kill them. This achievement will not count the enemies that are already dropping when you shoot them. They must be actually on the line. When you get to sections where there are many enemies on zip lines, kill as many as you can. This will be very difficult to get post-campaign or from re-visiting areas later. If you miss a lot, go ahead and just reload your checkpoint and try again.

Note: This must be done to different enemies. You cannot repeat this over and over again on the same enemy.