This one is as simple as it sounds, but it can take a while, especially if you’re not fond of mounted combat. The only way to earn kills towards this achievement is by couching your lance, which can be done by holding X by default on keyboard. Any kills with a lance outside of the couching stance won’t count towards this achievement.

You’ll find it much easier to kill light armored enemies such as archers while couched, so try to avoid heavily armored infantry and focus on the lighter armored troops first.