Superman and Batman are given one of their greatest foes by Hamilton, Swan and Klein in World’s Finest 142 (June 1964).
The story opens with the mysterious Composite Superman attacking the heroes, and getting the best of them. Looking like a half-Batman, half-Superman, but with green skin, the Composite Superman displays the powers of many of the Legion of Super-Heroes, which just makes the good guys even more confused.
We, but not the heroes, discover that the Composite Superman is really Joe Meach, a janitor a the Superman Museum. Brainiac 5 created perfect miniatures of the members of the Legion, which are on display at the museum.
Meach, unhappy with his life, had a chip on his shoulder against the much-loved heroes. In a sequence pretty much stolen from the origin of the Flash, Meach is standing in front of the Legion display when lightning blasts through a window, hitting both him and the statues, and endowing Meach with the powers of all the Legionnaires.
Superman, Batman and Robin try brute force and subterfuge, but nothing seems to work against a villain who is smarter, more powerful, and can do anything from turn invisible to split into three people. In fact, as the story ends, both Superman and Batman have been defeated by the Composite Superman. But he feels his powers wearing off, and leaves them, rushing back to the museum.
Meach is too late, and reverts back to his normal self, losing all memory of his other existence. Superman and Batman have no idea what happened to the Composite Superman, and just hope he does not return.
Bad news for them, good news for us, he does return in a couple of years.