Simply load up campaign mission 4 "Struck Down". There will be four times when you operate the remote sniper (the first time at the start of the level does not count as the gun is not loaded).
Time your shots, make sure that you don't miss, the enemies tend to stay still so precisely line them up, keep your reticle on them for a few seconds to ensure you will not miss. I tend to rely on the reticle glowing orange to make sure that I am targetting correctly.
Don't stress about taking out every single enemy, I took my time shooting and my teammates did contribute to the killing of the enemies. In the first section I only killed one enemy and the rest were finished off by the team.
Listen to your teammates, at the end of the 4th sequence I even stopped shooting when the enemies were still aplenty because Hesh was announcing they had left and I could physically see the helicopter flying away so I seized shooting.
I created this video to help:
Hope this helps