This can be grinded in memory sequence 7. You will need to head shot most, but not all, of the enemies in the first sniping section. Then restart from the checkpoint and go again, over and over.
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...