damn, it might be another bug*, do you mind posting a screenshot of the boss room (or sending it privately wherever you are comfortable doing so)?
The boss room is the same one you pick up the spell in, and once you pick up the spell the gate* to the left should lift and grant passage to the boss
Well, I figured out one possible reason for this bug. I'm fixing it now, patch should be life soon. while you will need to get the latest version, you might not need to restart, we'll see.
The problem i identified comes up when one changes scene (or exits game) between collecting the spell and fighting the boss. do you recall if that's what happened?