HUMANKIND

To win this achievement you only need to own one city at the end of the game.

You can play the game with as many cities as you want, but once you reach the end of the game, you need to ransack all your cities, but one and the achievement should pop, once you see the end credit story and as long as you are the winning player of course. I had armies in all my cities as defense and when I was ready to win the game, I ransacked all my extra cities and kept one.
Keep in mind that ransacking takes 1 turn so you should not wait until your absolute last turn. Do it a few turns in advance to be safe.

BUT once you take out your own cities, your FIMS will plummet, so be aware that all your income will drop. You could also use absorb city to do this, but that will give a big stability issue and costs quite some influence to do.

Quickest and easiest way is as I described: just play and win. A few turns before the end ransack all but one city. Play the final turn. Achievement should pop in the end game movie.