Late-game Town Enhancement.

Besides summoning, take Attack (Mass Acceleration), Defense (Damage Prevention, Resistance), Education (Magic Attraction), and Spellcasting (Mana Restoration, Arcane Knowledge, Fluctuating Mana).

I can't say for sure about the Orc yet – I need to think about it. But Logistics definitely needs to be developed for a long game. On large maps, you can't get anywhere without Logistics. Therefore, you'll have to give up something.

Added 36 minutes later
I can't say anything about how effective this build is, as I've never played to such high levels, but more or less against the Orc:


But! Only if the Necromancer has a Golden Horseshoe or other artifacts that increase luck. Luck is very important for the Necromancer. Since the damage is high, doubling it will be very useful. And defense is definitely needed against the Orc, otherwise the Necromancer won't last long. And also, hope that the Guild of Mages will drop a Celestial Shield or a Phoenix.

But this build has a significant drawback – big problems with neutral creatures. Therefore, I suggest option #2, which eliminates the problems with defending mines at the beginning.