Not sure if it's been mentioned, but going through 170+ posts is not easy.
Now They Fly? - If an enemy is driving the Warthog, jump into the passenger seat if it's free. You're now free to melee the driver and get a kill. It'll take two melee attacks to kill them.
So hopefully they'll hit a jump, so melee once before the launch and dot he second while mid-air. Achievement unlocked!