This achievement requires you to perform a melee takedown on an enemy while in one of the floor grates using the constrictor harness. If you didn't pick the constrictor harness as part of the story, you can get it in the Manhattan Bunker District. Luckily, you can get this achievement right after getting it.
Towards the end of the mission, you'll come to an area with an Übercommander that can only be accessed using a floor grate (there will also be another soldier in there with him). Go into the grate and come out where they can't immediately see you. I'd recommend making a save point right here. Fire an unsilenced weapon at the ground, then get back in the vent. The soldier will walk over to where you were and you can easily take him down with
. The achievement should pop right there, but if you want to get a few more to max out the perk, reload the save point and do that four more times.
Here's a clip of me doing this to demonstrate:
Hopefully this helps some people. If you see a mistake or have something to add, let me know and I'll edit (don't just downvote with no comment, that doesn't help anyone).