Since I was completing the game on veteran anyway, I thought I may aswell try for this achievement, and it's fine.
Like others have said, there are only 2 real areas of difficulty. So here's what I did;
When the soldiers are crossing the field, I hid proned behind the 2nd thick tree to the left of the field. The soldiers had to walk around the tree and thus me as well.
The other part that you may have trouble with (though I did first try) is when the 4 soldiers are dumping bodies. There will be 2 throwing bodies into the water, 1 in the middle and 1 walking around to the left. What I did was waited until the left 1 was walking away, then shot the middle one. After just shoot one of the 2 throwing bodies and McMillan will shoot the other one simultaneously.