Very easy, set difficult to easy and roll out with your drone, kill the first chopper with two grenades after you left the building, you can´t miss him, then just follow the route, a chopper appears over a wood constrution, he fly from left to right, keep calm and try to aim before he reaches the left/right side.
Then two MIL transport choppers come up on the left side, wait untill they start to drop troopers and kill them during this time.
Then a third attack chopper appears, wait until he stands in position and shot him up, achivement plops right after him.