it's the "Full Power" thing that leads me to believe that this is spite.
other than that, I'd say it would be a drawn out battle but with Galactus the eventual winner.
we know he can consume mephisto's domain thus mephisto's energy/power source can become Galactus' feedstock.
this would take mephisto out of the game whilst amping up Galactus to whatever fraction mephisto's domain is compared to the 616 universe on which Galactus usually feasts.
you misunderstand me. in the first battle [during "strange tales," way back in the day] dormmy did 'overcome' eternity, but lost his physical form in the process. in their the second battle it was both he and umar whom overcame eternity, and that battle took place off panel.
basically, neither of those battles can be used to accurately gauge dormmy's current power - as one took place decades ago, and the other was a shared feat.
He over threw eternity because of the shift in the Cosmic Axis, and he did is with the help of Umar..
Without the Shift in the of Cosmic Axis Dormmy looses bad.
Seriously, Dormmy is on par with Odin since they stalemated.
Galactus is out of his legue.
Depending on how much Galactus consumed he could win this, but with his average showings I could see these two taking him down all day long. I'm thinking he would need some help with these two.
His average showings--by the graces of plot lines that can make sense, as well as letting the feature characters (i.e., characters going against Galactus) continue to exist in the story lines--are almost always when Galactus is WEAK. Obviously, full power is very different. So the majority of Galactus' average showings (the ones where he is weak) are just irrelevant here.
If people have a problem with that logic...blame Marvel writers for always having to write Galactus down from his power set, which in turn happens so frequently that they become average showings, in order to make interesting stories and compelling drama. Otherwise, almost every story featuring Galactus would literally last (just like a truly-written Flash) 1 panel.