The Mangudai is the horse unit with a bow from the Mongol civilization. You start the mission with 36 of them. You need to micro a lot to keep them alive and use your other units as tankers. You can also lower the difficulty if you don't want to struggle with this one. I did it on hard difficulty, you can watch the video below to see how I did it.
Twinkle Hooves Achievement (The Battle Of The Kalka River Hard Difficulty):