Originally Posted by Dark Splank
You get the achievement for killing them all without them shooting back. You will know when they see you because the music starts. Took me a few tries, try on Easy.
Thank you, I got it. Seems that pausing and hitting restart level was stopping me get the achievement. Quit game, played level from the beginning (not continue option) and I got it first time by doing exactly the same thing I'd done previously. Not sure if anyone can confirm that or not.