Devolik
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After about 5 heroes. And the army isn't very large, twice the size of what usually attacks during the game. It's easily defeated while sitting in the castle. The main thing is to know the computer's priorities.
First, it will target the "Panthers" (I forgot what they're called).
Second, it will target harpies.
Therefore, give the Panthers reinforcement and that's it. You can easily defeat it. And if you don't have a Warrior, don't forget to put goblins in the tower.