Mephisto wants revenge on Galactus for their last encounter.
Scenario 1:
After plotting away he decides on the following course of action to annoy the hell out of Galactus:
He makes a contract with the Silver Surfer, similar to what he did with Spidey. History is retroactively changed so that Norrin and Shalla-Bal escape in a spaceship while Zenn-La is destroyed and Galactus is deprived of his herald.
He goes around and makes similar arrangements with the other heralds so that none of them are available to Galactus.
What will happen? Will the complexities of contract magic affect the likes of Galactus?
Scenario 2
What if style scenarios: Instead of doing nothing, Mephisto outright attacks Galactus at one of his weakest points, e.g. after Galactus has been punched unconscious by the Thing (FF #243) or maybe when Galactus has been defeated in Annihilation.
Can he finish Galactus?
Scenario 3
Similar to Beta Ray Bill's plan. Mephisto steals some planet busting tech from some advanced civilizations, planting it on various planets Galactus has targetted. As Galactus approaches, he blows them up (or if Mephisto he has the power to simply destroy planets under his own power he just does that).
Can Galactus stop him?
Scenario 4
He steals the Ultimate Nullifier either from the Human Torch in the past (returning from Galactus's spaceship), or Reed's lab in the present. This time he directly attacks Galactus with the UN. Does he destroy Galactus?
Galactus has been watered down and under powered so much that Living Tribunal should erase him from existence and find a replacement out of embarrassment
Scenario 2.
Could work, but under both circumstances the people around won't be standing by doing nothing, but could Mephisto kill a unconscious Galactus? Yes I would think so.