Kingdom Rush Origins

Mounted enemies can be found in the level, "Duredhel Outskirts" (one of the expansion stages, right of the world map). They're typically Twilight Avengers sitting on scarab beetles.

As only 13 spawn in that stage each time, you'll have to play the level a number of times to unlock this achievement as I believe they only spawn in this spawn in the stage. You can defeat them by any means and on any difficulty. I used this stage to also grind out the "1000 burning nuts" achievement.