Yes, this is an easy question to answer. While, yes, technically, I suppose you can 'kill' the Time Devourer, I don't think that's the 'canon' way of doing it, as it were. The point is, killing it will kill Schala too, and all that sort of thing. The only 'true' way of defeating it is by using the Chrono Cross. And that is something that Crono and his party, for all their strength, did not have in their possession: for all their brute force, it was a more deft and precise weapon that was to prevail.
And the only way that that weapon could be forged was via the splitting of time into two dimensions. Crono simply couldn't accomplish that sort of feat. But Belthesar could.
However, all that said, I'm not certain that Crono is the stronger. After all, does not Serge master Luminaire, Crono strongest technique, at a mere two thirds of his ability? True, he's not an innate sorcerer, as Crono is, but he does seem to have a mastery of elements that could outmatch Crono. As, indeed, he seems to be a superior physical fighter. Crono's ultimate weapon is a Rainbow sword, correct? However, though Serge can use a Rainbow Swallow, he is also weilder of the Mastermune, and in that way has an affinity for that holy sword even as Glenn did. He combines what is apparently sigaldrous power equal or greater than Crono, with physical force and weaponry at least the equal of Glenn, having possession of arguably the most powerful single weapon in the entire Chrono universe. As such, as much as it might be difficult to admit, I think Serge is probably the stronger of the two.
So, to counter what Zephira said, I think it's almost certain that Serge>Crono. Don't let nostalgia fool you, heh.
