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You must redeem the enemy's heroes yourself; this is the only way to complete this mission on a hero, and it is the most important thing to constantly pay attention to.
Another tip: don't deploy all your units in battle; it's better to leave most of your army in a mine and attack with 14 minotaurs (2 minotaurs per unit - 7 units). Because if you deploy all the furies, they will die from the first spell of the enemy, so let 2 minotaurs (or even better, 1) die instead.