Ok, I stayed up until 2am last night fighting this critter, spending nearly two hours straight in single battle until I finally won. I got the after text talking about the spiral key. Then the emperor appears standing with a question mark above his head and my quest helper points at him to "Speak with Emperor Yoshihito". Approaching the emperor gives no "X" to speak. Leaving the palace and reentering resets and I have to fight the Jade Oni again.
Not trying to be mean, but seriously! I'm a magus that's not even Mooshu yet but can still beat him alone. (I'm really close to Mooshu though) You must be pretty weak or very low leveled if it takes you that long to beat him. Otherwise, I don't know how to help you; sorry!