This can be obtained over multiple games but if you want to get it quickly I recommend the following:
Game type: Random map
Resources: Infinite
Civilization: Huns
Pop limit: 500
Starting age: Post-Imperial
3 players (one of them your Italian ally)
Other settings can be whatever you like but islands and/or standard or lower difficulty AI will let you train units in peace.
Build another TC and train vills until you have 20 or so and build tons of barracks (34 lets you queue up your entire pop limit at once). Once you have all the barracks you want, kill off your vills and train condottieris up to your pop limit. Delete them all and repeat and the achievement will unlock as soon as you train your 1,000th unit.