Once you're in the dungeon, choose to escape and use the chair as a shield. Use the left door, even if it says locked. If the option is grey'ed out, it's because you didn't accept their offer at the start of the story.
It's been almost 2 years since I played the game, so this information is just from me clicking through it a bit, so let me know if I'm right/it was helpful to you.
