I have found a bug in skirmishes. When you play the custom one and start it you can give commands to both sides of the army. And when after this custom skirmish you start a normal one, where you buy your units for gold and fight a premade army... but here the controls for controlling both sides are still present. So the challenge is gone if you use it.
But when you start normal skirmish without starting the custom one, the controls are not present :)
So if you want you can cheat at these skirmishes this one, but i don't thing its a good thing :)