SAMURAI WARRIORS 5 (EP)

Simply use Equestrian or Skills drills on a mount. Note, you don't have to complete on a single mount. The total number can be spread across as many mounts as you want. To make this easy, simply buy a mount then either purchase more after a level or gain them as rewards in Citadel mode. Then, select the mount you wish to train and then select the other horses in your stable to "consume" them to train that initial mount. Do it 20 times and the achievement will pop.