1) Play the game on very easy!
2) Choose any civilization and pick a small or medium map. (Better on islands)
3) Have only one AI Civilization against you.
4) No city states.
Now make sure you don't train any settlers or if you find any during your exploration in ruins delete it at once.
Also if you have the Brave New World expansion , you can choose Venice as your civilization and your enemy's too. The reason is Venice can build only one settlement, and that makes things really easy!
Beat the game on any way you know best. Fastest way is to attack but that's up to you.
Once you win the game the achievement is yours.
Good luck :D