In the JLA / Avengers crossover, circumstances permitted Supes to wield both mjolnir and Capt. America's shield simultaneously, but had he been allowed to keep them, who on his same level from Marvel or DC stop him with his knowledge of how to use them both perfectly with his own abilities added, and what level of being would it take to bring him down?
I doubt the shield would add much. Sure, it’s an indestructible (for the most part) object and he could throw it with enough speed for it to trim the top off mountains. But nothing he couldn’t do himself.
The hammer though - if I accept the classic interpretation that it will ‘possess the power of Thor,’ then that’s bloody interesting. Using (again) classic Thor, that would mean opening portals, high level elemental control, force fields, god blasts etc. It would make a significant difference.
Now, if it is modern Thor where (apart from a few writers who remember what Thor/Mjölnir is supposed to be capable of), the hammer would simply be a bludgeoning weapon that can make someone fly and summon lightning. In which case it wouldn’t be as important to someone like Superman.
Superman with magic resistance and endless energy absorption would essentially be unbeatable. Even you take away amping himself with sunlight, you need to go too high up to have a chance and at that level its meaningless discussions.
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