This Achievement is a little weird so I will explain it to the best of my ability. On the other guide it states you must get a 1 shot head shot but that is not entirely true, I got plenty of 1 shot head shots and there were no Action Camera moments. All the head shot Action Camera moments I got were from getting 1 shot head shots right when the popped out of cover and mostly every time I killed an enemy with a head shot when he popped out of cover. I almost every time got an Action Camera moment.
Strategy: The way I got a lot of instant head shots was to kill everyone behind the cover but 1 guy, then I would aim down the iron sights with my rifle at the exact spot his head would be the next time he came out and if I got the slightest feeling he was gonna pop up I shot and almost always got an Action Camera moment.