I don't see how I can survive the fight against the demoness when going into the spire dungeon with Naevy. I can deal damage, but the demoness has no health bar, and after some turns of receiving damage I just die and it's game over. Any hints on this?

Have you tried to use Silence spell on the demoness? Ryen can learn the spell if you build the lab and research Silence spell.

You need to remove the debuff "Poison Dust" she casts on you, or else the skill "Catalyst" she uses later will deal additional 1500 damage.
The health bar is invisible for some boss fights, but that doesn't mean that you can't win. ;)

Thank you, with Silence it indeed became quite easy! Did not need to remove any debuffs if I remember correctly in that case...