I did this on Normal difficulty,first attempt so its really not that hard.Use 'Dart overlay' as it highlights enemies and civilians differently,take it slow-if you pace it correctly most of the civilians will have taken cover or fled-then just pick your targets.
1-Use 'Dart overlay' the slow motion & enemy highlighting is key.
2-Take it slowly.
3-Obvious but pick your targets.
Whilst the other guide for this achievement suggests just running through or past the enemies-It is much more fun & rewarding to blast the enemies to bits & leave the bewildered civilians alive