Gelimer
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I've never tried it, but theoretically, why not use ghosts as a means to drain mana from the enemy hero beforehand? That is, we level up a secondary mage hero, give him ghosts and some other units, attack the enemy's main hero, use magic to hold out as long as possible, and drain as much mana as possible. And then attack with our main hero. I think this might work against a warrior with his relatively low mana.