I left it open to interpretation/choice as to what "at his best" meant, but it implied what was achieved rather than what might/could be achieved.
thus Galactus, in the BC arc, nearly consumed the 616 universe, and instead destroyed and recreated it.
this match, however, is set in the multi-verse, so Galactus wouldn't get an auto-win, because he hasn't been shown to consume a multi-verse.
also, if I'm not mistaken, there are Celestials in all universes. thus even if Galactus went about chomping on one universe after another, the Celestials would be free to pick off the rest of Galactus' team.
and of course, the fight would take so long that Galactus might even tire/"grow weary" and end concede...?