Worshipper – “Betrayal” [2nd campaign, 1st mission]

Yes, at the very beginning, you can find the Wanderer's Boots, which protect 10 monks. You don't need to rush to get them; you can pick up other minor items instead. My problem is different. By the end of the mission, I have a small army left, and I can't defeat the opponent. And at the beginning, they give me an army that's smaller than his! How can I kill him?