In an effort to consolidate the five guides all explaining the same thing but differently, I'm putting this guide up.
This achievement is bugged.
Not bugged as in if you do the things it asks for it won't unlock. Bugged as in the most random thing you do will get you the achievement. As proven by it's near 100% completion rate. This one unlocked for me on Harvest. I was Needlering two players, died, and killed one post mortem. Then it unlocked. Given the guides to this achievement. Looks like if you or your target are in the air when the kill is made, or you earn said kill post mortem, it unlocks.
The way you are supposed to earn this achievement is to be airborne from a ManCannon and kill another player. Wreckage has on ManCannon for each team and gives you nice air time. Though I would recommend softening up a target sitting where said ManCannon spits you out at before you go into it. Alternatively, ezmode way is to grab Rockets and blast someone sitting on the ridge while in midair from same ManCannon. Or wait near one and follow another player through it right after they do and assassinate them in midair.
It's also possible to achieve this on Shatter. On either base there are two Grav Lifts. These also count as "ManCannons." These are where I earned Clay Pidgeon and Special Delivery as well. Simply stick someone, dart into ManCannon and viola. Alternatively, if you drop into it from the upper level where the Grav Lift spits you out, you'll bounce up and down rather than get forced out. From there, earn a kill and again: Viola.
Harvest has Grav Lifts as well but they are buried behind where you initially spawn and on the ends of the greenhouses in the middle.