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If the hero levels are the same, Zoltan has a very small chance of deflecting a level 5 spell.
Necromancers don't have morale, and luck may not trigger (2 percent). The troops have low initiative. Therefore, both humans and dwarves can improve their luck, morale, and gather as many artifacts as possible that increase initiative – and then attack. It's a good idea to take Runic Harmony for the dwarves.
I used to be afraid to play against Necropolis. I always had fewer troops in battle. But then I realized that undead troops are generally weaker than living ones, so they can be fought.