Story related and cannot be missed.
Galactus is the boss fight of episode 8, “Taste of a Throneworld” and you fight him while piloting catering trucks. The goal is to shoot tacos and ice cream cones into his open mouth so that he will be too full to eat the planet. On hard, this can be difficult because you will die in 2 hits and your partner is usually already dead and will not have had time to respawn yet.
The tactic I used was to generally stay right in the center of the screen. This will give you time to avoid the laser beam swipes on the top and bottom and the arm swipes on the sides of the screen while making the minimum amount of movements. The reason I suggest this is because all his attacks seem to hit a rather large portion of the screen. Other than that, keep shooting him in the mouth when he opens it and he will walk away when full.