"The Downfall of Superman" - - *When Superman stages a wrestling exhibition for charity with famed wrestler Antonino Rocca, Mr. Mxyzptlk appears and gives Rocca greater strength than the Man of Steel, enabling him to defeat not only Superman, but also Hercules and Samson, who Mxyzptlk summons from the distant past.
Witnessing this, gangster Duke Marple forces Rocca to use his new-found might to retrieve Marple's hidden loot which had been sealed in a cave by an avalanche. When Marple then attempts to murder the wrestler, he is astonished to find that Rocca is invunerable to his bullets. Then Superman, Samson, and Hercules arrive on the scene and prevent the crooks' getaway, using powers of electricity and magnetism.
The entire scenario had been arranged by Superman in order to discover the hiding place of Marple's loot. Superman and Roccca had exchanged identities to make it seem that Rocca had super-strength, while Cosmic Man and Lightning Man of the Adult Legion of Super-Heroes had posed as Hercules and Samson, and the phony Mr. Mxyzptlk had actually been Superman's pet, Krypto, his voice supplied by his master's super-ventriloquism. *Source: The Official Legion of Super-Heroes Index #1 - Published by ICG.
Also features the back-up story: "Superman Under the Green Sun".