Corptain in Yo-kai Watch 1

After upgrading the Watch to Rank S, talk to the man at the bottom of the left room on the second floor/first floor of the Gourd Pond Museum in Breezy Hills to hear a rumor about the Old Mansion. Go there and enter the Side House, and interact with the folding screen to reveal stairs leading to the attic. There, you can fight Corptain once every day until he's befriended through the regular method.