Select Very Hard when playing Arcade Mode, play through all the matches then beat Galactus and the achievement is yours.
A recommended team would be Iron Man>Dr. Doom>Chris. This is a projectile spamming team HIGHLY recommended for the galactus fight. When fighting the 2 shadow characters before Galactus, spam Uni Beam (QCF M) and they will go down easy. When fighting Galactus, you want to be in about the middle of the field, and spam your Projectiles (QCF M with Iron Man and Dr.Doom, QCF H with Chris). If he's about to use one of his huge attacks (He'll go into the background and start building power when he's about to use one) use X factor right before he launches it, it will negate all the chip damage you would have taken (Assuming you guard his attack). Use Iron Man's QCF L+M ultra when you have the bars to use it, just make sure he's not about to attack, otherwise you'll take a big hit.
All in all, the hardest part is getting to Galactus, I recommend simply using your best team if the Projectile Spam team isn't working for you, then try to beat Galactus and if you lose, switch to the above team.
NOTE: A great substitute for any of the above mentioned is Hulk, X factor+ Gamma Tsunami (QCF L+M Hyper Combo) will obliterate Galactus', or anyone else for that matter, health.