I just picked up on this again after not playing for awhile and I'm in the diet building. I got the whole place mapped out and I'm pretty sure I beat all the bosses, but the door on 6F is locked. Can anyone give me a clue here?
You probably already went there. Take a look at your inventory: you might have an item called "Yahirono Himoragi" in the key items section. If you do, go to the Temple of Amala and go inside the central pyramid. If you can't, then you should head to the Obelisk.
I think that should solve your problem.
EDIT: And by Obelisk, I mean the Tower of Kagutsuchi that drove the Obelisk into the ground. If the Obelisk is still around and intact, you're not that far into the game yet.
Solve a man's problems with violence, help him for a day. Teach a man to solve his problems with violence, help him for a lifetime! - Belkar