**SOLUTION FOR THOSE WHO HAVE COMPLETED THE GAME**
First off if you haven't played the game yet use the other guides posted and gain this achievement it will save you some trouble.
If you pass the Shanty Town/Gondola sections without gaining the achievement you will have to use this method once game is complete.
I had completed the game and realized I had missed this thinking another playthrough was required I read some comments on here from Ryozenzuzex
to which I give full credit for helping me and hopefully others in the same predicament...
Fast travel to Helicopter Hill in Shanty Town, go to here on the map to be exact http://www.flickr.com/photos/94355597@N07/8589697748/in/phot...
You Should be looking out to tower above something like this http://www.flickr.com/photos/94355597@N07/8588597223/in/phot...
(If zipline isnt where it is on picture you will have to climb tower to make it so) jump on rail in front of you and to the roof behind jump on zipline and use the rope ascender (hold x) to get to the top.
Enemies should spawn (If they don't re-enter Helicopter Hill I could only get enemies to spawn in the daytime) http://www.flickr.com/photos/94355597@N07/8589697352/
turn around zip down to where you were and enemies should follow I found the rifle works best to knock them off the line but weapon of choice is up to you, once they're killed run back to zip-line ascend again enemies spawn again and repeat until achievement pops.
Cheers to SiebeTolsma who used this guide and was kind enough to make a video version