Well for one thing, Onslaught wasn't a reality manipulator until he took control of Franklin Richards. So unless the OP states it, Onslaught = Xavier + Magneto.
And even if he uses the reality manip. version, i don't see why they couldn't beat him. Martian Manhunter is one of the most powerful telepaths in comicdom, and Aquaman is pretty good at it too. I remember in the Onslaught Saga Cable said he and Jean could protect people from Onslaught......and they're both crap compared to MM and Aquaman.
If he turns into a psionic entity, i don't see why GL couldn't contain him in a forcefield. He's contained supernovas and all kinds of energies in there so.....
I also don't see why GL couldn't just absorb him if he wanted to. He turns into psionic energy. Ok, it's psionic, but it's still energy. The GL ring can absorb it.
Hold on, i agree with most of the post above me, but no way are Cable and Jean crap compared to MM and the waterboy. If anything, i'd say it was the other way around. MM is the only one who can be mentioned in the same breath as these two. Waterboy>>>>Jean?? GTFOH!! Waterboy>>>>Cable? GTFOH!! SMH..... MM= Jean yes. MM>=Cable, surely. But neither are crap compared to him. Physically they may be crap, but mentally, otherway around. True Story. No heat though on the GTFOHs.