What mixed hero class of the (Necropolis) faction do you like?

I agree with Celt. From Death Knight to Demonologist, and from Demonologist to Archmage, by studying Haven magic. Where infantry can't pass... the Archmage will crawl on his belly!

I agree that the Demonologist is a good choice, but you could also consider a Dark Priest, at least because of the vampiric attack, so he won't be killed right away, or a Lich, but the Demonologist is better, summoning demons + 50%.