anti-Necropolis faction

I thought the same thing... But no. You need to silence and interrupt the summoning. Otherwise, you'll get wrecked. And you'll get elementals (if you're lucky, not fire elementals)... And the resurrection will be quite powerful... So, the best anti-Necromancer is either Haven or Demon, I think everyone will understand why. Especially why Demon.

In my opinion, Demon is far from the best option.
Firstly, mass debuffs are very unpleasant for the Demon, and he won't have much mana left after them. (Let's not assume the opponent is an idiot. He will make sure he has 100+ mana). And let's not forget about banishment.
Secondly, Darkness doesn't affect Necromancer's spells; 2/3 of the spells don't work.
Thirdly, the Necromancer develops faster at the beginning.