How do I defeat the Spawn of Rovagug at the bottom of the Tenebrous Depths?

I'm going to be honest, I played this part on Story difficulty (and the Spawn still downed my main character).

This same question was asked on both Steam and reddit. One of the common tips is to buff your party (especially with Haste). Another suggestion, unsurprisingly, was to summon monsters and/ or elementals. If your summons don't occupy the adds, you'll probably want to deal with those first.

There are a few videos on YouTube. This one does the fight on Challenging difficulty. The first 1 minute 39 seconds are spent summoning and buffing.

I have faced the same problem as yours (with the same character) and maybe I've just been lucky but I have beaten him quite easily, despite losing a couple turns and having several characters be less than useful in that fight.

I didn't focus much on buffing since the spawn has a dispel attack.

I rushed the

aiming at killing them as fast as possible. Sword Saint Magus, either tank Valerie or Amiri (can't recall) and Ekun focused fire on them, while Octavia cast sneak-attack AoEs on everything hostile and Kaessi created a mud deadly earth below the spawn, a little behind and to the right. Jubilost stood to the side, waiting to be able to focus fire on the spawn (there's not infinite bombs, using them just to deal a little damage that the spawn easily heals is not useful at all) but if you need him for taking out the other enemy in one torn, so be it.

Then, I placed the other melee in front of the spawn, my Magus at the top (in order to force the spawn to only target one at a time with its cone attack) and kept unleashing arrows, bombs, fire rays and mud long range kinetic blasts (which almost always missed) in the Spawn. Ekun got possessed but he had the freezing bow and targeted Jubilost, who had good cold resistance.

Jubilost used force bombs, because I had not taken holy bombs. Holy bombs work better, as does Smite if you happen to have a paladin available.