I dont think that Onslaught can tap into Franklin's full potential, yet, if he gets celestial level he would beat Thanos, that goes without saying. That is if the link between Onslaught and Franklin is not touched during the battle.
To be honest, I think that any smart uber-powerful character would just pwn Onslaught by severing his link with Franklin, after that he would just go down like a chump. Thats the truth of the character, without Franklin he is a chump -just in comparision to the average cosmic level.
Originally posted by Lord S
Thanos vs. Onslaught (Xavier+Magneto)...I'd lean toward Thanos. Not cause he's Thanos, but because his durability and offensive firepower are a force to be reckoned with. Onslaught at this stage had trouble getting by the X-Men in their initial battle...barely being able to put down Bishop. Thanos would eye-shot half the team.
Thanos vs. Onslaught (Xavier/Magneto/Nate/Franklin)...no contest. Onslaught murders him ten times over.
Someone mentioned Onslaught would mind rape Thanos...how remarkably absurd, considering Thanos has psychically used Xavier as a tool in the past.
