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Listening to you, it seems like the game needs to be completely reworked because the balance is off.
In any case, demons (with players of equal skill) always defeat necromancers (and therefore, not just them).
What gives demons such an undeniable advantage?
The necromancer's army is numerous, the hero's defense is high, the hero's turn efficiency is extremely high, and the spiritual connection provides some protection against demons (at least compared to many others, such as dark elves or humans).
The only argument that comes to mind is the low initiative of necromancers. But if you have good skills, I don't think there can be any talk of an undeniable victory for the demons!