In case you all forgot, the greatest part of Galactus' character, as well as something he rarely uses, is The Ultimate Nullfier. In his hands, the Nullifier itself can wipe away an entire timeline from beginning to end. One wave of that bad mutha and Phoenix would be done for. She may be capable of destroying a universe with all of her might, but the Nullifier can wipe away the pasts and futures of entire timelines with ease. And the Nullifier is actually an aspect of Galactus, so it's only fair he gets to use it.