Composite Superman (Real name: Joseph Meach) is a fictional supervillain who first appeared in World's Finest Comics #142 (1964). He is one of the strongest enemies that Superman and/or Batman have ever faced
Joseph Meach was a diver who had fallen on hard times. In an attempt to draw publicity to him, Meach set up a water tank on the sidewalk in Metropolis and dove from a building. Unbeknownst to Meach, the tank was leaking and Meach would have died if not for Superman's intervention. After learning about Joseph's misfortunes, Superman obtained a job for him at the Superman Museum where Joseph was employed as a janitor. Meach grew angry towards Superman, and being surrounded by artifacts of Superman's career only grew his anger. One night Joseph was sweeping in front of a series of statues depicting the Legion of Super-Heroes, a bolt of lightning went through an open window and hit the statues. The statues (which were lifeless duplicates of the Legion of Super-Heroes) unleashed an energy blast that hit Meach. Meach discovered that he got the powers of all the legion members depicted.
As a villain
Determined to defeat Superman, Batman, and Robin, Meach used his new shape-shifting power to turn his skin green and form a costume that was half-Superman's, half Batman's. Dubbing himself Composite Superman, Meach left a letter to their bases to meet him on a mountain and told the heroes that if he was not allowed to join them, he'd reveal their secret identities, which he learned via telepathy.
Composite Superman then created situations for Superman or Batman to handle, which he sabotaged to humiliate the heroes, then sorted them out himself. When Superman and Batman deduced Meach's evil intentions, Composite Superman forced them to retire or else he'll expose their identity. Clark and Bruce fake their deaths while hiding in a lead room using robots, with which intended to see if Composite Superman was ready to let them die. Artillery shells, which were made with kryptonite, were fired at the robot "Superman", making it seem as if he was falling to his death. Then, robotic replicas of Batman and Robin asked Composite Superman to help, which he refused, after which the Batplane they were flying in was shot down.