Please Note - The description of this achievement is deceiving. You must kill your target while they are mid air after being launched by a man cannon or gravity lift, not just shoot them.
This achievement can be completed on any of the Crimson maps where there is a man cannon or gravity lift, but the easiest place would be on Wreckage.
Wreckage has two man cannons that converge to a single location that have roughly a 2 to 4 second airtime when launched out of them. So using any precision weapon can get the job done with a steady aim.
Harvest and Shatter have gravity lifts that take less than a second to go through. Using the sticky detonator or a plasma grenade (or a well placed rocket or railgun shot) would work on these maps, but means the achievement is more of a hit or miss as there is latency in other player’s connections to consider.
If you choose to try and complete this achievement on Wreckage as I did, here are some things to note.
1.) The platform that both man cannons converge at is located in front of the initial spawn for the energy sword. The energy sword spawns here only at the beginning of the game, so take note of its location before someone picks it up.
2.) The energy sword and landing platform is closest to red team’s initial spawn meaning they have the advantage of taking this position first. This also means blue team is more likely to go through their man cannon as it is more in their direct path at the start of the game and after re-spawns in a CTF game.
3.) Red team spawns and re-spawns facing their man cannons launching end, so getting a kill on a red player for this achievement is less likely unless playing King of the Hill as there is a hill point at the platform landing. King of the Hill would force red team to re-spawn at locations that would put the man cannons right in their paths.
4.) DMR, BR, and LR can get the job done, but require hitting every (or most every) shot. Sniper rifle, binary rilfe, or railgun would be the easiest precision weapons for this kill. Using the energy sword (as it is up there already) could work as well if you jump at the right time and location to get the kill.