Best way to deal with ethereals en masse?

[Master Shake]

Ethereal undead die to banishment just like all the other undead. Priests, priests, and more priests. Treat priests like a normal unit. Indeed, they are less expensive than many elite units, and being sacred, cost half the upkeep. I just finished a successful campaign against Ermor as Ulm, and I had about 10 priests for every normal commander, and as many as 20 per province. Just get some stable high defense units up front, and your priests will wear away at those armies.