Superman's Pal, Jimmy Olsen #112

Jimmy Olsen is taken to the small nation of Duxania to try and discover the secret weakness of the super-villain Magnaman before he can kill the Duxanian king. He seemingly does so, and is dubbed a Duke of Duxania by King Otgar. It turns out they've been duped about Magnaman's "one weakness," but Magnaman accidentally perishes when his super powers unexpectedly wear off in the middle of a complicated hoax in which he disguised himself as Superman and had himself buried alive.