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Find a city state that has a heavy cavalry unit, ally, levy, upgrade to tank, kill the tank.

Details:
Normal game where there are many city states (can start in Classical era, can use barb clan mode).
Train and save 3-10 Gaesatae. Turn them into corps and armies later in the game.
Tech to Oil and Tanks.
Save some envoys if you don't control all city states.
Find a city state that has a Heavy Chariot, Knight or Cuirassier.
Ally the City State, levy its army, upgrade to Tank next turn.
Prepare an ambush in neutral (non allied) territory (bring cavalry, tanks, field cannons, general/himiko)
Bring the Tank into the ambush and declare war on the city state.
Soften the Tank and kill it with a Gaesatae (regular, corps or army).
Remember the tank vs spear memes of Civ 3.

City states rarely build heavy cavalry. But they do.
Worst case scenario you have to wait for other Civs to tech to tanks and war them.
You can still unlock the achievement after winning the game.