You can only do it on the first nations from the game (europeans).
The easiest civ is the Spanish where you need to reach lvl 55 of your metropole to unlock all cosmetics of your home city.
You need to create a Skirmish game, type Nova & Orion 5x to give you 30.000xp and tuck tuck tuck to finish your enemy base with Tommynator. Each won will level your home city by 2 or 3 levels. Rinse and repeat