You've beaten the 8th end-of-chapter boss, but haven't finished Ruber. There must be something more. As usual you can't save before talking to Merlin. Unfortunately this time when you finish talking to Merlin the game doesn't auto-save. Worse, you're dropped right into a boss fight with Ruber, so you can't save on your own either. Hopefully you have a recovery heart on you.
(Talking to Merlin refills the energy from the last battle so you start out ok at least) To fight Ruber, do so like you would an ordinary knight. Just don't let him get over you for any reason. In time, he will be defeated.
[Bug] From here on some text is displayed starting left and above where it normally appears.
After saying some agree things, Ruber's energy will refill and gets the spin attack. If he starts this attack, back up some. Its range is a little larger than his normal slashes. Beat him the same way you did last time.
Once you beat him... he says more angry things and you must fight him again! A full four boss fights with no chance to save! I went berserk here slashing him but his energy never went down to zip. I accidentally beat him when... he used his spin attack, hitting the center stone on the screen. Maybe this was in the movie, but I never saw it so I wouldn't know. You may have to hit him a few times in this last fight before this will work, but I don't think so. Just lure him and let him off himself. You win!