There's basically two ways to get this one:
1) get the killing blow on someone who is mid-air.
2) knock somebody off a map edge with your Whip Shot.
The first way requires patience, good aim, and a sprinkle of luck. The best way to try this is to target enemies who spend a lot of time in the air like Doomfist, Genji, Junkrat, baby DVa, Widowmaker, Pharah, etc. Wait for them to be extremely low health (your Whip Shot doesn't do a lot of damage) and then smack them out of the sky like a righteous fly swatter.
The second way is easier than this, and just requires playing on a map with edges and finding an enemy who likes to hug the perimeter. The Whip Shot has a decent amount of knock-back, so just boop them off and you're golden. Regardless of how you unlock this, once you do you'll get the achievement and the adorable cute spray for Brigitte!