Any advice on how to beat it?
First hero - Valkyrie or Paladin or Death Knight?
Valkyrie - allows you to attack better. Ramps up quicker.
Paladin - Defends better. Searches ruins better.
DK - Early on, you are getting attacked by a lot of units. Hurting them is better than helping your own units, so Chaos is better than Leadership.
Some things that I have noticed:
A couple of units (like Light Cav or scouts) in the rear, wandering around and retaking villages, confuses the Orcs and makes them waste turns chasing down one Light Cav with a stack of 8.
The elves aren't that big of a help.