Since they adjusted the achievement list for this game it has become super easy to complete. This was a really ungrateful move to all the people that wasted hours on the terrorist hunt achievements in the first two years. This solution is no longer applicable please see Pending Demise's solution
An easy way to get this achievement is to protect the windows in a protect the hostage terrorist hunt. Find windows that you know they will repel up. Board these up and shoot a small hole in the board with your pistol so you can see if there is a zip line outside or not. Then hang around the vicinity of the window so you can hear if the terrorists deploy zip lines. When they do so go to the window you heard them deploying their line and watch as they come up. This is a very easy place to get headshots and it takes quite a while before the terrorists react. This is also good for your xp as you get a headshot and clay pigeon bonus.
Locations I found to work well.
The best I found is the cigar room in Kafe Dostoyevsky go to the piano and board up those two windows. I think at least 8-10 come up during a full round of defend the hostage.
In the consult room in the Consulate they will also repel up those two windows though you are quite exposed to the main door if you teammates don't cover it well.
In Hereford base if you defend the second floor they will repel up into the kids bedroom.
On the Kanal mission if you defend the control room they will repel up the server room next door. It is easy to shoot them from the server room or you can shoot them from the side in the semicircle window area in the control room.