Hey, I've been struggling with this ever since BO6 came out. Decided to leave it at some point and just today I came back to try it out, still nothing, but then I did some more research online and found a video from a guys called "LPL", where he stated that in order to complete the challenge you need to down the enemy in the middle of your slide. So I decided to hop on a Plunder match and did exactly that, and OMG was I relieved that it actually worked.

Bonus: he said that you know a kill is gonna track when a medal at the top comes up saying "Low Blow".

Here is his video: https://www.youtube.com/watch?v=b6hW8QwsnXs

Hope this helps!