This can only be completed in memory sequence 7. You will need to complete each section of sniping, without killing a single enemy. It is strange what constitutes a kill in these sections, so just don't aim for the head. Restarting from checkpoints is allowed
Here is a video guide with English commentary for this achievement!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKCPof8gLhv...