Some MCU characters are extremely powerful whether they are villains or heroes. Celestials on the other hand are powerful cosmic beings that can literally create and devour universes. They were present long before the Big Bang and are the oldest race to exist. Defeating them is no mean feat but some of these characters have done so in comics and the others might just be powerful enough to do it in the future.
The mad Titan, Thanos, was one of the most powerful beings in the universe when he wielded the Infinity Gauntlet. He destroyed multiple planets and civilizations alike. As a matter of fact, in Marvel comics, Thanos successfully defeated two celestials even when they were hurling planets at him. Moreover, his will and determination is his greatest strength that allowed him to acquire all the Infinity Stones in the first place.
We are familiar with the Phoenix Force as it inhabits Jean Grey in the MCU. Phoenix is a cosmic entity that is the the epitome of all psionic energy that ever existed. This entity is powerful enough to swallow stars and annihilate universes. Moreover, the Dark Phoenix has unimaginable power. The Phoenix force is also immortal and indestructible. So it wouldn’t be a surprise if it defeated a Celestial.
Galactus is literally the Devourer of Worlds. He has even taken on four Celestials and defeated them by absorbing the powers of planets. After all, he can even bring back life to otherwise dead planets. He is also the oldest living entity of the Marvel Universe. Once he tried to entirely eliminate the Celestials by devouring every planet that they impregnated with their seeds. Eventually Galactus became arguably the biggest threats to the Celestials.
There is barely any one as smart or even smarter than Reed Richards. The interdimensional council of Reeds that is made up of a hundred variants of Mr. Fantastic himself, had battled celestials and even used the power of the Ultimate Nullifier. He could easily construct a machine powerful enough to defeat celestials. As a matter of fact, he used his intelligence to recreate the Multiverse after the Secret Wars.
The King of Inhumans and ruler of Attilan has unimaginable powers. His voice is one of the most powerful weapons in the universe as his whispers can destroy armies and town while his screams can destroy cities and even planets when powerful enough. Even a slight sigh can kill innumerable people. He has battled Thanos and Galactus with his voice and delivered some faltering blow. Above all, he has defeated a host of Celestials in an alternate universe.
Scarlet witch is one of the most powerful beings in the comics. She has both the powers of the Mind Stone as well as Chaos Magic. She can rewrite reality, resurrect the dead, and annihilate all mutants. As a matter of fact, she has also defeated the Dark Phoenix. The MCU in the future can explore her powers further as she is actually extremely powerful in the comics and could definitely try to defeat a celestial if she was truly determined.
Thor has previously defeated Apocalypse who was blessed by the Celestials. They had given Apocalypse Celestial armor and made him their Avatar on Earth. However, Thor the God of Thunder defeated him with his sheer determination and his power. Despite Apocalypse almost breaking Thor’s spine, his ego and pride forced him to keep going. He turned to dark magic and finally defeated him.
The Guardians have already defeated a Celestial in the MCU. Quill’s father Ego was a living planet and Celestial who wanted to use Quill to further his kind and eventually take over the world. However the Guardians defeated him by bombing the planet and destroying his “heart”. It took around 6 people to do so but was an impressive feat nonetheless.
No character in the MCU can truly be underestimated. Dormammu, an ultrapowerful interdimensional entity was defeated by Doctor Strange in the MCU. Thus, characters in Marvel can definitely defeat Celestials if they truly give their all.