We have never see the full extent of Onslaught's powers and the scary thing is, he bitched slapped the combine efforts of the major players in Marvel and rocked the marvelverse. Seeing how you have the two most powerful mutants.......Nate and Franklin, inside him, he's in a league of his own. In addition, he has the combined powers of Magneto and Professor X. This guy is a reservior of psionic energy. Added with more Nate's telepathy and Franklin's reality warping powers, he'd atomize them in a heartbeat. This battle easily goes to Onslaught. Sorry boys.