Easiest way to do this in no time is:
- Go to Forge on Bloodline
- Modify the Gametype to have 10% Damage Resistance, Shield Multiplier to No Shields and 500% Weapon Damage Modifier.
- Start the game with 2 players (can be done on splitscreen)
- Have the secondary player to grab the Gungoose in Forge Monitor mode and board it with the main player in Player Mode (use
to switch modes)
- Have the secondary player to lift it up and rotate it until the Gungoose is facing directly at the Forge Monitor and the main players crosshair turns red.
- Finally hold Fire
with the main player then simply have the secondary player to switch into the Player Mode as well
and you will kill him midair with the Gungoose and pop up the Achievement.
I hope this helps!