Turn your difficulty to Very Hard after you have picked your lineup and progress through the game. You get unlimited continues so don't worry if you die at any point. You want the timer to be at its lowest, so you can simply run out the time while you are chipping out the enemy CPU. You also want the damage to be at its highest for when you face Galactus, as the sooner you can kill him the better.
If you're struggling with beating any rounds, I would suggest using the following characters:
- Hawkeye (Assist Type (Quick Shot (Greyhound))
- Doctor Doom (Assist Type Hidden Missiles)
Using these two characters with either Sentinel or Hulk will be good for immense zoning, spamming Hawkeyes Quick Shot (Greyhound) (+) while using Dooms Hidden Missiles assist will keep enemies away while chipping them out and it you decide to you can combo Hawkeyes Gimlet (+) Hyper after a hit confirmed for additional damage almost guaranteeing your victory for the round.
Just like in vanilla Galactus is more or less exactly the same as he was. After defeating the heralds you will be faced off against him, try to preserve your meter if possible and just use your basic jumping > > to deal good damage on him without the fear of being punished from one of his damaging attacks.
Once you have 5 meters or when you’re left with Doctor Doom on the field go ahead and break out your level-3 X-Factor and spam his hyper Sphere Flame (+), the move is a upward ball of energy that rains back down on the enemy, the amount of separate beams from the attack will do a lot of extra damage to Galactus killing him off much faster.
Just like in vanilla, one notable move Galactus has is a grab move that cannot be blocked if it is range (the grab will be obvious as he spreads his arms out in the background and looks as if he is about to slap his hands together). You need to super jump to avoid it otherwise it will hit, guaranteed.
After you have brought him down a bit more in health he will start to use stronger attacks, lasers than can instant kill any of your characters in one touch. If you have it use X-Factor at this point to block and avoid massive chip damage and then proceed to finishing off Galactus.