Can Necromancers even be defeated?

well, he also has restarts and blocks, besides ranged attacks; he can easily take down Necropolis. I think there's a small chance in the mid-game, when not every Necromancer has a couple of angels yet, if you manage to find some powerful magic around weeks 2-3, like an explosion.
because it gets worse later, but I think smart Necromancers will eventually figure out how to counter it.
(
maybe limit Necromancy for all heroes to the advanced level? because right now, they're too powerful.

or, as an alternative, ban the Transmute spell, at least; that way, there will be fewer skeletons. I don't think the creators anticipated players converting 1-2 level creatures from all castles into skeletons.