Inferno tactics.

Could you increase magic power and use an explosion of skeletons to counter?
Or persistently search for the black orb and use blindness/berserk?
Or develop troop speed for attack and overwhelm with the entire group (frenzy?)?

This is all good, but let's not forget that a Necromancer can have all of the same things, plus a bunch of skeletons on top. And who will get that first? To answer the question, we need to think about where artifacts come from. Artifacts are obtained from strong guards or in utopias. And who will break the utopia first? I think we don't need to continue...

And also - has no one ever defeated Inferno with Necromancers?

You can win against anyone and with anyone. The question is only about the odds.