I was playing homm3 complete with my GF and decided to play with inferno.
I planned to farm demon with marius as my starting hero but on the very first day i figure out that Isra (necromancer specialist) was available. The idea of using him to farm demons as well as skeletons quickly came to my mind and i tried it but unfortunately the map was really creepy and boring and i did not have the chance to really do it.
I'm not the best at heroes but I don't think that would work very because of the severe morale penalty to your army for using undead. You could get leadership which isn't a bad skill but there's certainly better ones. Plus Inferno just isn't a very good faction, and Necro really misses the vampires and dread knights.
^ my dead proposal thread. Maybe I'll update it again someday
^ My novelization of HoMM 5