I don't think Atum has the power that Shuma has. Or, the power of some other Elder Gods and demons. But, I do think that Atum IS the "silver bullet" so to speak, for many of the most powerful beings in creation.
In actuality Atum was able to absorb all of the ancient deities and their offensive abilities such as force bolts. He would then grow in power and devour the Gods themselves.
Yeah, I think it would be cool to see an Atum series, where he travels through different universes and dimensions, fighting nothing but these cosmic class, dimension ruling horrors.
I'd give Atum a good shot, ad this kind of thing is his bread and butter. Particularly, he uses the same tactic that Strange used successfully against SG, mainly eating his enemies' mystic power.
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I am going to have to go with Shuma-Gorath. Atum had trouble with Set and was unable to even contain Thor. Shuma-Gorath is on a different league than those guys.