If Onslought really wanted to win, all he'd have to do is use the same magical "writers" powers he used throughout the entire saga to create massive loopholes and inconsistanies. He'd simply write them all out of existance. The only reason ever he was ever beaten in any aspect is because he got bored of owning everyone.
Huzzah for Onsloughts stupifying effect on the writers of Marvel!
If the laws of Marvel were kept intact during this fight, and somehow Onslought and his opponents were written correctly, it'd would be this:
Onslought uses telepathy to control/trick superman and hulk to team up with him vs Juggernaut. The Juggernaut would handle things superbly (being he is immune to fatigue and cannot take physical damage PERIOD) until they decide to go for a psionic attack and begin going for his helm, at which point he's in trouble.
Somewhere in there ol Juggy might lay supes or Hulk out for a moment and try to break the others neck, probobly supes since hulk just gets angrier and it becomes difficult to keep him still for it (almost happened once, but not quite).
No matter how you slice it, someones going to be toast before Onslought is the new target of the fight. If they get ol Juggies helmet off and Onslought fries him psionicly, the other two don't stand a chance against Onsloughts psionics, and he wins. If Juggernaut SOMEHOW pulls it out his rear to kill of supes AND the Hulk before his helm gets knocked off (I'm a fanboy of Juggy but that's most likely impossible), then he gets to square off against onslaught and since Onslaught can't break the rule like he did during the hype faze of his saga, Juggy will win if he can keep his cap on, seeing as how Onslught will get tired of any useless physical victories, and Juggy can't get tired.
Basically, all hope rests on Juggernaut if Onslought can't break the rules, which by definition, will never happen, heh.