Kill The Burrows!
Burrows are the big weakness of the Orcs. If you can get in and kill their Burrows they will no longer have food to build their army. This will also reduce the defenses of the town. Ranged units work the best such as Archers with Improved Bows. Any unit can work but it can be more difficult with melee units if the town is built correctly.

[ Click to Enlarge - 199 KB ] Cast Faerie Fire on the Tauren and Grunts. This can make them much easier to kill.