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I made sure that the Earl wouldn't move before the hero, so I deployed 3 units: two archers with 500 units each and one adept. At the beginning, I cast Berserk on the Earl; it lasts for two turns, then I cast Subdue on the nearest unit to my units, the Highlanders. On the 3rd turn, I cast Subdue on the Earl again (he still didn't attack) and blocked their path with the remaining dwarves. After that, I cast Berserk on all the dwarves, so they couldn't reach me and attacked each other. While they were fighting each other, I raised the Earls from the dead (not resurrected), reducing their hit points to 1. At the end, I cast Subdue again (the duration was 7 turns for me). In one turn, 6000-8000 Earls were killed (ballista and archers). I had 240 mana left, about 70, so it's better not to take the second group nearby right away.