Firstly, I would like to say you can kill enemies in reflex mode and still get this achievement. If you would like the boost in points (but gain in frustration) you can turn off reflex mode in your options menu before starting the mission to get a point bonus. But reflex mode makes this easier, so I would advise to leave it on. But you must complete this mission without being caught and raising an alarm. Here is a video from 360gameTV (credit is given to him/her for video) to help you out: