You will need:
- Someone who is willing to be suzerained by you (hint: a fake account).
- That person (fake) must have enough resources to make 400 cities (the max needed).
- That person (fake) must make a temporary alliance and declare themselves as HOSTILE against your alliance.
You attack the city with your fastest hero and 1 scout.
You win, and the suppression begins.
At this point your Colonize Achievement registers that +1 colony has been done.
The "fake" person then deletes that colony, and the hero and scout return automatically.
REPEAT :-)
The downside to this is, that the abandonned city remains there, as is, until the server maintenance is performed, when it is then removed from the map and becomes a flat again.
For optimum (cheapest) results I suggest taking low level flats and making these into the cities on subject.