Play the game type Air Assault on the map London Calling and spawn in the Airship. Shoot 5 planes out of the air, simple as that. As of the writing of this guide, planes that destroy themselves by ramming into the Airship count as kills with the Airship. I was able to rack up 22 kills, half of which were planes ramming themselves into the airship and dying. The game mode on this map is two rounds so each side has a chance to spawn in the airship.