Help with Romancing Saga 2 Revenge of the seven heroes.

I've been playing this for the last few days, really enjoying it but I think I've just bricked my save file. Can anyone help?

Spoilers below: I've beaten the first hero, and have done the time skip. I've set my new team up and was exploring and started doing the quest about succession. All good so far, until I found that crashed/landed airship and spoke to a guy and basically got tricked into being a slave on it. So I completed the dungeon and got to the boss, Bokhan, and he has a giant puppet that destroys my team in two or three turns. Stupidly I saved over my main save file just before the boss room, and I can't fast travel out of the airship (map says travel isn't available) and there is no exit on the ship I can walk to. I've tried and tried against this boss and I can barely damage him, let alone beat him. Does anyone know how I can proceed?

(Yes I know I'm a moron for not having another save).