After reaching level 48, in order to earn my next big spell card, they told me to go to Shirataki Temple and defeat Plague Oni. I went in and fought all the battles and so forth, but Plague Oni never appeared in the temple. We fought every battle up to and including the WaveBringer, but Plague Oni didn't appear. What gives?
Aftr you have killed WaveBringer, go intside the Temple. There is a boss here, so kill him. Directly after you have killed him, there will be a text telling you the Plaque Oni has appered. Now kill him. :-)