There is a way in doing the challenge but u got to be quick. As long as you can snap as many before u get to wer they spawn a lot. When u get there juast after the cut scene wer the heli comes in, pending on the wer yr looking, look for a set of steps in the left hand corner with something that looks like a stall. If u stay ther in camo mode a lot of enemies comes down from those steps and uf u stay hidden u will be able to do the challenge no problem.