Prime gets to sit in the center of the sun for a day prior to the battle, and also gets the power gem from Marvel, a Red Power Ring, and gets to wear Wonder Woman's bracers.
I'm sure if this was the Hulk with the power gem people would be saying Hulk would beat Galactus and then smash the Living Tribunal for the hell of it.
If Prime actually uses the Power Gem to its upmost capabilities, ie. all the power in the universe that was, is, and ever will be, he might kill someone to death here.
Galactus really just needs to shut off the Red Power Ring to get a massive advantage in this fight. Superboy Prime missing his heart and practically all his blood would be quite the upset.
Superman sundipping is bad enough and now your giving Prime a day long sundip with murderage and power gem.... right Galactus Jobbing aura needs to be turned off for him to win. Unfortunately it got stuck in the on position during the crunch.
It's really sad to see what Galactus has become. He is kind of the measuring stick for a team/character. If they beat him they are now either a multi-versal/universal threat or a major badass to be set up to fall against the next major villain. It's like
[insert new badass hero]:hi big G I kicked your ass now i'm hot stuff.
[insert next major villain]: Hey new badass I'm the new big kid on the playground and lookout other heroes I took out someone who took out Galactus. FEAR MY POWER
[Other heroes] : GASP OH NOES!!!! LOOK HOW STRONG HE IS.
Assuming both opponents know the capabilities of each other, Galactus really just needs to shut off/destroy the power ring to cripple SBP beyond belief.
And I thought the general consensus on these fights were PIS doesn't apply so this is a competent, non-jobbed, almost full-power Galactus taking on Superboy Prime.
Prime has resisted attacks with red sunlight quite a few times. In fact, he beat a Pre-Crisis Kryptonian to death AFTER flying through a red star. The sundip, and the red ring would probably add even further to his resistance. I'm sure the red ring could provide shielding as well. Not to mention as soon as it happened, Prime would fly as fast as possible away from that. And, not only that, but I doubt Galactus WOULD teleport a red sun on top of Prime anyway.