It depends on which form of onslaught. His original form which Thor had a good crack at would be taken down. Added to that Onslaught had trouble using his psycic powers on aliens. With Superman being an alien who has vast psycic resistance that removes one of his greatest powers.
w3rd(wait a second, I did this inside a thread once, superman least favorite, onslaught most favorite, onslaught owns)
__________________ Huey: The truth hurts, Jazmine. The world is a sad lonely place and no one gets anything for free, you know what? One day you and everyone you know is gonna die.
I'm sure if DC ever put them in a battle, superman would win by some gay azz technicality (wouldn't wanna upset those nerdy *** ultra superman fans who think superman is undefeatable by anyone)
__________________ Huey: The truth hurts, Jazmine. The world is a sad lonely place and no one gets anything for free, you know what? One day you and everyone you know is gonna die.