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Dude, Onslaught beat the crap out of most of the Marvel universe. Even in defeat he was obnoxious.
That having been said: If by Sandman you don't mean Wesley Dodds, any of his brood or the idiot from Spiderman but instead mean Morpheus, King of Dreams, I think there's a good argument to be made that Onslaught is really all just a bad dream. (Many people would argue that the saga in the comic sequence should have just been a bad dream. *ZING!* -Rimshot-