Fire Arrows (Ench 4) is the "more affordable" ethereal-killer, as it gives magic weapons to all your archers
. Other "area fire" spells work well too, but a Nature-9 Jotun can't get anything in Fire however
...well, if you've access to Astral (Utgard) the always-useful Star Fires work, but if you were you'd use Etherealness yourself I suppoe
!
With IW or Niefelheim frankly I don't have any idea except building magical weapons ... What ethereal stuff are you fighting ? If they aren't magical/mindless you can try to ruin their morale, etherealness doesn't help, if they are big baddies you can try to swarm them with Vaetti or Imps for example...