Mephisto, relative to his entire career, fought Galactus during a time period where he was wildly more powerful than is normal. He still lost because Galactus threatened to /literally/ eat Hell lol.
Odin is garbage compared to Celestials. In his first encounter against a single Celestial, he channeled his own power + Zeus' power + another less powerful Skyfathers energies into a single bolt. Said bolt harmlessly bounced off the Celestial. In the Destroyer armor + with all of Asgards essence, he fared better. Even managed to cut a limb off a celestial. But that ultimately was nothing but an annoyance as the limb was healed and Odin soundly defeated.
Galactus meanwhile can flat out kill a Celestial. Big G can often stalemate 2 or 3 random Celestials and be a legit foe to the bigger bladder Celestials 1 on 1.