Dark Horse requires you to win AH-ladder (i.e. finish at top-1 position) while never finishing in top-10 of all the previous ladders.
The challenge is pretty simple and suits a lot to a Farmer playstyle: you avoid getting into top-10 on all the ladders but rush forward in the end to try and make a top-1 finish in AH-ladder.
It sounds tough but actually is quite possible since AH-ladder usually requires a lot more than 10 rankers.
Ideal strategy would be finish AH-1 (AH minus one) ladder at top-11 position to start growing power earlier.
Winning is pretty much the only goal of this challenge.