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There is a bug/exploit in mission 4. If the Cave Goblin units from the hero's specialization eliminate all enemies with a Fireball spell at the beginning of the battle, and the battle ends, they remain and are added to the army (interestingly, this doesn't work in battles against demons). Out of curiosity, I completed the mission using this bug, and by the time Duncan arrived, I had 10,000+ Cave Goblins.