Well, Galactus isn't Hulk. Galactus wields Power Cosmic which makes him god-class being.
How did Tony Stark overcome the power and invincibility of Power Cosmic? How does his Galactus-Buster Armor work?
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Unfortunately, there isn’t currently an answer. Since it only appears at the end of a video game (Marvel vs Capcom 3: Fate of Two Worlds), all we know about the suit is that, after the defeat of Galactus, Tony Stark used the data from the battle to make the Galactus buster armour.