After 4 trys the game can’t unlock the 2nd Step of the Mastery 
Sorry I can’t read german, would you mind translating the quest?
defeat 5 enemys with cavalry before they are in the 2nd age
Sorry for my bad english skills
Oh can the mongols even produce cavalry at dark age currently?
yes
I am also stuck on the same mission and i have tried killing more than 5 units with Scouts(Townhall/Stable) and early horseman(Stable). It is so frustrating as i have tried multiple times and it doesn’t count when these units are in fact cavalry.