This isn't actually that hard to get, as long as you're careful.
Just go into a new game with an ancient era start, and barbarians on.
Make a bunch of slingers,and use them to attack some barbarians to get as much xp as you can, and put it all into improving their defense. Promote at least one slinger to level 3. Once they become obsolete, put them deep into your territory, and keep them asleep until you unlock rapid deployment and have built at least 2 aerodromes with hangars. Move a slinger to one of the aerodromes, and airlift it. Voilà.