Ummm...don't know too much about Onslaught. Just know that he is a wrecking machine.
The duo of SS and Gladiator is a very formidable team. However, could stand to chance that Onslaught might still be able to take them.....perhaps even easily.
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Re: Onslaught vs Gladiator & Silver Sufer
If Mindless Hulk can damage Onslaught's armor, Gladiator and Surfer should be able to do the same and once the armor is off, I'm betting that Surfer could disperse or transmute Onslaught's energy.
The problem with this battle is that it could become Onslaught and Gladiator vs Surfer (if 'Slaught can break through Gladiator's mental defenses). The team can take it, however, if they are quick.
IIRC Gladiator has a very potent mental defense system. That being said, if Onslaught is at the "beginning" of his evolution, the two can stop him without too much hassle most likely. The fact that Gladiator's incalculable strength level is psionically based only helps IMO.
Now, Onslaught's evolution wasn't really explored that much. We do know he was using Franklin and Nate to evolve, but we don't know to what extent their assistance helped him in doing that. Either way, since (assuming) this is only Xavier + Magneto Onslaught, i'd say the two would most likely win.
Onslaught at the beginning of his career would be dead after an interesting fight.
Onslaught at the end of his career kills them both.
__________________ The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts.
I wouldn't be too confident about that (pun intended ). There's no way to know whether his psionic power could or could not be contained by some sort of power cosmic construct or anything.