For this achievement; after you have picked your 3 characters, change the Arcade Mode settings to the following:
Difficulty: Very Easy
Time: 30 seconds
When facing Galactus and his Heralds, I would try to dispose of his Heralds as soon as possible while not using any of your Hyper Meter. Try air juggling and remember that they share a life bar so you don't need to hit them both together.
When Galactus arrives, double jump and hit him in the head as much as possible. This will avoid most of his attacks. When his energy is about halfway down, you should have enough for a Hyper Combo so call in both your partners together ( + ) and unleash hell with all three. Do NOT do this is Galactus is about to crush you or unleash beams from his fingers though.