Defeating a demonologist

[Kamamura]

I have this very interesting game on a large world, empire. I play as senator, and my only remaining foe is a demonologist.

I control most of the cities, but there is one 4 squares large in the center I cannot get a hold on.

The problem is, while I have a nice income, two weapon masters and churn basic troops and train them to elite level, my foe has like 3 major armies with about 10 demons and chaff. I have found out that he is quite undefeatable - no matter what I hire, it has magic resistance of 4. The best unit against demons available to me is the occassional archbishop, but he falls quickly to a fireball spell. Augurs are worthless because they use fireballs, and so are wizards.

Any idea how to defeat a demonologist? I have a feeling that once he grows powerful, a warlord can't do much to stop those demonic SCs. I tried to starve him from city access (they need people to sacrifice, right?), but it does not seem to have any effect.