On the mission Harpurs Ferry I tried out the above solution but I went a little further...
Without even moving and some good timing its possible to get not 4 head shots but 9 if you also kill your allys,
300 head shots can be a slight grind so I became "that guy" and wondered if ally head shots counted too which to my surprise they did!
To further prove the theory I actually got the achievement on an ally head shot.
once you kill all enemies and allys just restart the level until it pops, hope this helps get it faster!