Gyoshi

I'd like to avoid spoilers, but I'm at a loss as to what to try besides starting over.

I got the Zangetsuto and tried facing Gebel again, but couldn't do anything but kill him and get the game over screen. (I got the 'new master of the castle' achievement, though I'm not sure what exactly it is in English.)

The game is hinting that I need to use less shards. So I tried selling all my shards (that you can sell) and unequipped them all, but that changed nothing...

Do I have to start a new game where I use/gather less shards before facing Gebel to progress? That's the only lead I have because I don't know anywhere else I could explore