Most people, self included, encounter this at Level 4. Even then, one has all of the right tools available to them, it's just a low blow.
Smoulderburn is a unique Will-o'wisp with 37 AC before it casts Shield (+4). Shield protects it from the singular spell it is not immune to. It has 124 HP and DR 5/magic. No character has a magic weapon at this point. Characters are only going to hit it on natural 20 or with a high roll while using the True Strike wand. Even then, that's a lot of HP to chip through. Triple the normal will-o'wisp. It has 4 sorcerer levels on top of its 9 aberration. Scare can be devastating. While it's reasonable to have at least 1 Remove Fear going in, he casts it repeatedly. Smoulderburn also has unique AI where he regularly switches targets--typically after casting Scare. Most people will not have access to communal resist energy. I had two L2 resist energies memorized. Even still, he has +23/23 attack and does 3d8 damage. Owlcat gave it Improved Initiative and Weapon Finesse.
It was intended to be a kick in the pants. That's only a problem for virgin Ironman players. I have no problem with designers intending the player to reload for certain encounters. People don't skip content for the same reason they don't skip parts of their food at a restaurant. Even with the context clues, it was intended to "saddistic game design" in Owlcat's own parlance. Why you are falling on your sword to swear this encounter as being something else, is bizarre.