Interesting strategies, QM! I'm not sure they are the best for the purpose, however.
I would think perhaps fire-blessed Vans or Sidhe or anything, really, could work, since they should be able to hurt the spirits and in the case of these two and some others also have a lot of survivability against centurions. Also, vans can sac slaves to spread dominion, right? That could be useful, too, if the game prolonged. Lightning bolts should work fine against centurions, as long as they aren't especially protected.
Shouldn't sauromancers be more effective skeleton spammers than spectators? Thusly, one would think fighting fire with fire would work. (Not that I know if spectators were used for that purpose
) Also, base C'tis has these serpent dancers that have magic weapons, and a high def and an above average MR; they should be formidable against the spirits. Especially with some bless and a skeleton spam to soak the hits.
Fire/Astral flagellants would conceivably work, as they should have enough MR to resist many attacks, as well as well enough offensive capacity to beat centurions.
Perhaps the best bet would be fire (+a bit of earth, maybe) blessed white centaurs; you get the supply and the blessing from the dryads, as well as cheap banishment. Also, temples are easy to make with Pan.
Or maybe I'm missing something? Certainly these would have a hard time, too, but I wouldn't necessarily focus solely on mages and priests under circumstances like this.