He attacked Earth while he was well fed in an Exiles arc. It took all of the superheroes from this Skrull gladiator prison to even get Galactus' attention, then the Exiles' Thunderbird went all-out, tore through Galactus' armor, and threw a special bomb Reed Richards designed in, destroying Galactus. Keep in mind, however, that Thunderbird was enhanced by Celestial tech (and the Celestials are arguably more powerful than Galactus), he'd physically beaten the Hulk into submission on a previous world, and he basically killed himself in the process of sneaking up on Galactus and breaking through his armor.
