Whether in turrets or by using your sub-weapon machine gun, aim for the head. Upgrading the Skill "Mondo Vision" early on will help immensely with this achievement, as it causes you to automatically zoom-in on your target as well as they appear to slow down. There is no in-game counter for this achievement, sadly, but it will eventually unlock. There are various Sub-Missions and Episodes that are good for grinding this achievement out, namely the Weight Limit one.