My favourite universes only have about one major character each, so, why not this universe? Composite Humanity can lift 537,500,000,000 pounds, run at 100x the speed of light, take 1.1 billion bullets in the same place and not have a broken bone and has 600 billion IQ. Also, if you add up other things, can yell at 565,000,000,000 decibels, which can destroy over a trillion universes in a massive black hole or space-time warp, has a body heat of 259 billion Celsius (and can therefore survive that) and can punch with 930 quintillion newtons of force.

Round 1-10): Possibly the most unfair fight ever. Composite Humanity can speedblitz them all, except maybe the Flash (Speedforce), who would burn up if he tried to touch us. In fact, the entire planet would turn into plasma.

Round 11): He'll regenerate, but Composite Humanity will kill him over and over.

Round 12): Is Darkseid here? Like full Darkseid? An Omega Beam might kill Composite Humanity or mess us up, but actually we are probably faster than that.

Round 13): We can scream so much louder than Black Bolt, who made Thanos flinch, that he would literally explode. Along with the rest of the universe.

Round 14): The scream would kill him. A quadrillion universes.

Round 15): Galactus has been beaten by weaker heroes before, even if he is fully fed, he will probably die if the shout hits him.

Round 16): Might be able to do something with his spells, but he probably can't handle the speed.

Round 17): He has his anti-humanity repellent, and manages to use it, but unfortunately he is only one human while Composite Humanity is more than one. He loses.

Round 18): Not sure, it might be able to win. Depends how fast it is. Whoever is faster can destroy the other one from reality first.