I couldn't beat the superboss Playful Darkness in chapter 3 because I didn't have anyone capable of targeting her low touch AC besides Ember, nor did I have save DCs high enough to nail her down and lower her actual AC with my crowd control spells. Meanwhile I found the chapter 4 superboss laughably easy in comparison. Though I've heard Playful Darkness can actually be cheesed by abusing the environment - I've seen people talking about how she can supposedly be attacked out of range of your party by the area where the group of Succubi are.
I've seen Playful Darkness get mentioned here a couple of times - in Owlcat's defense, I'll say he's actually doable without having to resort to specific spells or Touch AC (at least on Core - haven't gotten to him on my current Unfair yet).
Hitting his 68 AC is almost like a pop quiz on the resources you have available to you and how to stack them all.
The easiest and most readily available tool for this fight is Finnean the Talking Sword, which I think almost everyone should have (notice the 14 AC swing with GMW). All the buffs in both cases are pretty readily available - the one somewhat out of the way buff is Guarded Hearth (Nobility Domain, grabbed via Impossible Domain). But Sosiel has Touch of Good by default, which will also give you an equivalent or even higher sacred bonus to hit (just clunkier to use).
I was talking to someone on Reddit about this fight, and we figured out that Playful Darkness is classified as a Monstrous Humanoid, so you need to actually use Instant Enemy on him with Aru.
This kind of goes back to my earlier post about the 2 major methods to dealing with difficulty in this game - "Brute Force" vs. "Figure out the Puzzle". IMO it's both the best and worst parts of the difficulty design in this game. Because they haven't invested in a sophisticated AI, or included enough complex encounter design, bloating the enemy stats is the most common way Owlcat ramp up the difficulty in encounters.
The toughest (and my favorite) fight in all of WoTR is also the one fight where I thought Owlcat really did put in the effort in the encounter design. Sadly those are pretty rare in WoTR. It was a particular battle in act 4:
The "secret" fight you get if you follow and figure out the Mysterious Runes sets across the city. It's one of those fights where IMO it's a properly designed encounter, not with a single boss with bloated stats, but a huge mixed team of enemies that target your various weaknesses. You are absolutely outnumbered, and the enemies are incredibly mixed (archers, demons, alchemists, kineticists, all targeting your various defenses.