I apologized for my mistake about Galactus' durability. However, I still believed hat Bill have a higher chance in because many haxes. Like I have already stated resistances to reality warping and weaker aspect of it-- which constitute to all most if not all strongest Galactus' power (reality warping including its Weaker aspect of reality warping: matter manipulation, time manipulation). Moreover, Bill even perhaps the most powerful hax in for any low 2C tier:
Higher-Dimensional Manipulation( a power usually reserved for all 1-A and few 1-B(
Umineko no Naku Koro ni) that could rival or perhaps stomp conceptual manipulation "aka Arceus" at tier low- 2C if one got a LOT of experience "aka Bill: over a trillion years old". Also,
Higher-Dimensional Manipulation can be used in theory to ascended in higher dimensional mid-fight which should be banned. An example would be Bill using this to become an infinite 4 dimensional being "aka At least a 2A" (with enormous advantages) or even into a 5 dimensional entity - "a High 2-A" in which case Galactus would be comparable to 2D 10000 soldiers in a paper )(harmless to 3D being) to 3D person where that person, while being principle of directly interacting with the 2D soldiers, can interact with all 3D construct (paper) where the soldiers exist and rip it apart with no effort. Following this example, Bill, a 5D being, would just have to destroy a 5D construct where Galactus exist to win with no difficulties.
P.S read
Higher-Dimensional Manipulation for elaborations
And even without his OP ability, Bill could still win overall got a higher attack potencies: :At least Low 2-C>>>> Low 2-C, and much much more abilities that make him easily win because of its significant experience differences to Galactus's where he is at least 100x older than Galactus: At least trillions years > just older than the universe.
Thus, to conclude, Bill would still win because of sheer 100x EXP differences to could make his abilities seem haxes(i.e. could mindfuck Galactus), his OP hax: Higher-Dimensional Manipulation, and also higher attack potencies.