The Dread Crypt of Skogenby is a tough little dungeon, no doubt! But I have seen groups take on Haathor-Vash and win. Most just didn’t do it with violence.
The idea is to create a world that lives and breathes without the characters. Some things are going to be beyond the characters’ powers, and it is incumbent on the players to recognize those situations. In many ways, later versions of D&D got “safe.” Encounters got designed with the characters’ levels and abilities in mind. In theory, characters could defeat anything they encountered in those games because they were level-appropriate. In Torchbearer, like in older versions of D&D, you will occasionally find yourself face-to-face with something that you just cannot kill without major mojo. In those instances, your wits really come to the fore. You can’t kill it, but you could run. You could try to trick it. You could converse politely. You could try to sneak.
Also, with regard to Skogenby, consider that Lawful characters might not consider the adventure a success unless they banish Haathor-Vash and free Jora, but other characters might see it completely differently. They might be satisfied with scoring some loot and never encountering Haathor-Vash at all. I’ve seen others get on board with what Haathor-Vash is selling. From her point of view, the adolescents of Skogenby are tomb robbers that stole from her and disturbed her slumber. And she’s not wrong. Maybe a little old-fashioned eye-for-an-eye vengeance is just what the doctor ordered? And let’s be honest, you could do worse than having an undead demigod as your patron. There are lots of ways the adventure can go down!
Incidentally, the one group I’ve seen that did manage to take Haathor-Vash down with violence? They didn’t go directly into the crypt at all at first. They waited for Haathor-Vash/Jora to come out and wreak more horror upon Skogenby. Then they rolled a boulder in front of the entrance to the crypt. When Haathor-Vash returned, they followed her to the secret, rear entrance (which they did not yet know about) and ambushed her there while the sun was coming out. After subduing the girl, they worked the banishment in the sunlight where HV’s guardians couldn’t protect her.