How do you counter flying units?

[kyrtuck]

Played as the Greenskins, and was having a right proper time smashing the gits until I came across a Beastmen horde that had a bunch of Harpies that done walloped me.

How do I fight back the harpies?

[Elitewrecker PT]

Harpies are small melee flyers, so any of your anti-infantry units can deal with them once they engage.
In the meanwhile you can jsust shoot them.