Superman, Shazam, Wonder Woman, Thing, Hulk, Colossus, Thor, Hercules, Wonder Man, Scarlet Witch, Invisible Woman, Namor, Silver Surfer, and Cyborg could hold him off until the smart people of the teams like Batman, Reed, Xavier, Cap, Iron Man, etc fix something up with the help of all the Gl's and Quasar to destroy Big G.
They'll be fu<ked if they destroy him. Abraxas will rise and then proceed to wipe them out. Unless the Phoenix Force is there. Then again here are 3 scenarios will likely happen:
Weak and Hungry Galactus:
They more than likely have a chance of taking him. Heroes win 8/10. Fanboy didn't specify the rosters, but if it is full rosters, then Galactus has no chance. On one side will Phoenix, and the other side will be Spectre. He was a JLA member at one time. Draco said it. With these two, 10/10 win Heroes.
Normal Powered Galactus 50% power:
Again Without Spectre Phoenix, 5/10 Heroes or Galactus
With Spectre & Phoenix 10/10 Heroes win.
Full Powered Galactus 100% power:
Without Spectre/Phoenix 9/10 Galactus destroys earth or whatever planet their on, and the ones who cannot fly or survive in space die instantly. Those who aren't invulnerable or nearly invulnerable die from the blast. More than 90% of the team are wiped out. Which leaves the likes of Superman, Sentry, Thor, Superboy, Martian Manhunter, Possibly Wonder Woman, A couple OF GL's, Quasar, Possibly Wonder Man and Iron Man... Ummm.... Sersi maybe... Um a few other heroes. But that's about it. A fully powered Galactus written right is only a notch or two below the Abstracts.(Death,Eternity,Oblivion,Infinity) He could possibly be on par with those Abstracts when fully powered. No way the heroes win without Phoenix or Spectre. What is prep time gonna do? He zaps earth, boom, no more earth. Handful of heroes VS. Galactus... Heroes lose.