I'm guessing you are in the same boat I was. You have to do a bunch of quests in MooShu before you are able to defeat the Oni(s). I had a quest to defeat an Oni before I was able to go in the MooShu dungeon for it because I hadn't done much questing in MooShu. So keep doing MooShu quests and you'll eventually be able to go into the place you need to! Have fun!