From Halopedia, the Halo wiki
Airborne is an achievement in Halo: The Master Chief Collection. This achievement is in the Halo: Combat Evolved multiplayer and is obtained by launching an M12 Warthog into the air for at least 5 seconds after which an enemy is splattered.
Start a custom game on Death Island with three controllers (and ideally, three people to control them). Set both the time limit and grenades to 'unlimited'. Once the match starts, two players go to the top of the map above one of the bases, one in a Warthog and one in a Ghost. The third player stays beneath the same base.
The player in the Ghost parks it at the end of the base structure with its back to the beach; this Ghost will act as a ramp for the Warthog. The player who parked it moves toward the front of the Ghost and prepares to throw a grenade. The player in the Warthog drives toward the ramp at top speed. At the precise right moment, the player at the Ghost throws a grenade (your choice: frag is more powerful, but plasma is less likely to skitter off target) launching the moving Warthog up the ramp and the Ghost, over the edge and tumbling to the beach below. The player who has been patiently waiting below will do their best to position themselves beneath the falling 'Hog, getting themselves splattered and unlocking the achievement for the driver.
In the alternative, two players can attempt to unlock the achievement by instead creating a larger ramp out of a Ghost and a Scorpion tank. Though timing is no longer an issue, the Ghost has a nasty habit of dislodging on contact and ruining the launch attempt.
Failing to sufficiently launch the Warthog by either of these methods will mean the Warthog will be airborne for less than the full five seconds, so even if the splatter happens successfully, the achievement will not unlock. The achievement will also fail to unlock if any part of the Warthog touches the ground before splattering the other player, or if the driver is ejected on the way down.