Dead Pool is perhaps the most versatile character in the history of Marvel Comics, mainly thanks to the fact that he can break the fourth wall and seemingly bend the universe to his whims through 'cartoon logic' – even if he wants to do something crazy like kill Galactus. However, Wade Wilson actually killed Galactus at one point, and he didn't use any meta abilities to do so.
In Deadpool kills the Marvel universe #4 by Cullen Bunn and Dalibor Talajić, Deadpool continues his crusade against all Marvel Comics characters. In this story arc, Deadpool realizes that he and all the other Marvel characters are nothing more than works of fiction who can never die, despite never actually being alive to begin with. This thought drives Wade Wilson completely insane, so he seeks to free his fellow fictional prisoners from their shackles by killing them all. And, in this final issue of the four-issue miniseries, Deadpool sets his sights on Galactus.
The issue begins with a series of characters dying/committing suicide inexplicably and without a hint of fight or self-preservation. It is then revealed that Deadpool is controlling them all through puppets created by the Puppet Master. When the Punisher kills the Puppet Master (thinking he was Deadpool) and Deadpool uses the Punisher puppet to end his life, Wade Wilson then takes a Galactus puppet to do the same to the Eater of Worlds. But first, Deadpool uses Galactus to kill some other cosmic powerhouses, like Nova, Silver Surfer, and Thanos, before finally killing Galactus himself.
Deadpool uses a Galactus puppet created by the Puppet Master to kill the Eater of Worlds
Deadpool's methods are shockingly brilliant and can be used by anyone
When Deadpool is facing an enemy as cosmically powerful as Galactusfans might assume he would resort to the “cartoon logic” that Deadpool is famous for utilizing in certain stories. It's not like Deadpool could actually stand a chance against Galactus, as the Eater of Worlds would atomize him with the Power Cosmic before Deadpool could make his first move. However, Deadpool does not use 'cartoon logic' to defeat this enemy, nor is he atomized. Instead, he comes up with a brilliant way to take down Galactus.
If the Puppet Master could create a powerful enough puppet of Galactus, then there would be nothing the Eater of Worlds could do to combat his influence. Galactus would be forced to do whatever the person controlling him ordered, including what he did in Deadpool kills the Marvel universe – that's killing other cosmic characters before taking your own life. While this happens in a comic book that is essentially a 'What If?', this is something that could happen in the main Marvel continuity, and could be done by anyone with access to the Puppet Master puppets – not just Deadpool.
This isn't the first time Deadpool has been a headache for Galactus
Deadpool becomes Galactus' annoying herald in Deadpool Team #883 by Skottie Young and Ramón Pérez
Deadpool seems to be a pretty consistent pain in the ass for Galactus in the Marvel Universe, not only because Wade knows how to kill him, but also when Deadpool wants to align himself with the Eater of Worlds. In Deadpool Team #883, Deadpool becomes the Herald of Galactus, and he angers the cosmic assassin of the Endless World. In fact, Deadpool ends up being fired, as Galactus can't bear to be around him for another second. The horrors of ending trillions of lives? That's easy. Being around Deadpool for more than a day? Impossible.
Although Galactus found Deadpool unbearably irritating when Wade became his Herald, it's fair to say that Galactus would find this preferable to being killed by Deadpool in a way that eliminated the possibility of fighting back. In fact, Dead Pool discovered Galactus's weakness, and not only is it effective, it's something that can be used against Galactus at any time and by anyone at Marvel Comics.