Works great in Sequence 6 - Memory 2. At the very start of this mission you must investigate 3 weapon crates. Go to the middle one. You'll find a haystack here, jump in from the building above. Two guards are standing by the haystack. Another will walk past it after half a minute. The last two enemies are patrolling the area, whistle to lure them to you. If you are unable to get five in one go, the achievement is cumulative and will count any kills you make, even if you reload the memory 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-HeW6XkKdUK2zqCoVSQZ...