Арсений
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Find the Sphere of Vulnerability, which removes all immunities from creatures, and berserk skeletons – it’s devastating.
Build a larger army than his.
Eventually, deal with Necropolis early or mid-game. By the end, almost no one has a chance.
Here, I disagree with you. The main disadvantage of Necropolis is bone dragons. If you compare Black Knights and bone dragons, the Black Knights will always be stronger (+ they have a double strike ability). If the computer doesn't build Black Knights but builds bone dragons, then it's still uncertain how the map will play out...
P.S. Although I agree with you about the Sphere of Vulnerability.