Age of Empires II: Definitive Edition (PC) - Achievement - Cuman Flush
Win the second Kotyan mission "The Battle at the Kalka River" without advancing to the Castle Age.
This achievement might sound more problematic than it actually is. Follow the instructions until you obtain a base and get your economy in order. Your allies will prevent you from being flanked for quite a long time, giving you the time you need to defeat Yellow.
They have two Castles and a few walls, but nothing a bunch of Battering Rams won't sort out. Just make sure you have a decent sized force to protect them with as bear in mind - their units will probably be more advanced, so you'll have to play the numbers game.
Overall, the resistance they give is minimal and the only thing to keep an eye out for is the occasional light attack which you should have the forces to counter.