Ok.....Ive done this argument many times, so Ill just get down to the nitty gritty, and all my arguments will be at current powerlevels (well..we havent seen Onslaught in a while, so Ill use his powerlevel then of course). Im discounting supermans ability to pull a superpower (or new powerlevel) out of his ass to help him defeat anything he comes up against.
Onslaught was NEVER destroyed. There is still the possibility that he will return. And it took a majorly pissed off Hulk (who has the potential to make superman look like a three year old etheopian in the strength department) to even crack his physical shell, which only made him stronger. And I doubt even Hulk could have done that unless Onslaught was busy with all the other heros. Im not convinced Superman could even crack Onslaughts shell. Why, you ask? Well, Onslaught got his physical power from the Gem of Cytorrak (Juggernauts source of power) and this is Magic. Superman has NO power over magic. When it comes to magic, Supes has nothing but a regular old human-level defense to it.
So how would Supes defeat Onslaught? You tell me, cause I sure as hell cant see a way.
*sorry for the choppy argument....Ill be back later for any quesitons on my logic, I gotta go take a grumpy*