"In honor of our fathers" - "One Khan, One Clan" [Campaign 3, Mission 2]

Learn "Frightful Roar" and "Wrath of the Horde" (the latter isn't strictly necessary, but it won't hurt). After defeating the brown garrison and capturing the brown castle, buy reinforcements and head towards the green castle. Leave these cities for the turquoise and purple factions to plunder (they might even fight each other), but be sure to buy out all the troops beforehand, leaving the enemy with "empty" cities. I've completed all the missions on "hero" difficulty many times – this mission is one of the easiest.