Best way to deal with ethereals en masse?

Easiest way to deal with masses of Ethereal units is to use Blade Wind. If you're Jotunheim that will not help though.

Alternatives: Anything with alot of attacks, preferably ranged (woodsmen, villans, etc) will null alot of the advantages of etherealness. By using swarms of ranged they will usually get hit.

2.) If you are running a 9-Nature Blessing, you can probably take a Herse or Jotun Jarl and give them a magic weapon (if they are undead, try using weapons with high attacks (Sword of Swiftness), if they are not, use wraith swords, no access to anything like that, use something that has a secondary effect)

3.) Mash with Gygja's with Dust to Dust (if they are undead) or Shadow Bolt/BLast (if living).

Nothing else I have really off the top of my head for masses of ethereal units.