Suggested by iKnowledge Randall Flagg has returned, but now he is fighting one of the strongest Justice League members out there. Martian Manhunter has a great amount of strength at his disposal. He does have a weakness to fire which could be dicey since that’s the kind of trick Randall could pull odd. Still, the overwhelming gap in physical abilities should be enough to give MM the key advantage here. No way Randall will be able to stand up that. Martian Manhunter wins.
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Consider this: https://www.deviantart.com/keyblademagicdan/art/Black-Adam-vs-Martian-Manhunter-665998246-What's to say that Randall Flagg won’t have the same effect? And Black Adam is a “grayer” character than Randall Flagg. He may survive getting beaten by the Martian, who I bet could fit the Burj-I-Khalifa straight from the ground, but that to be kept in mind.
I think the difference here is that Adam at least believes that he is just so the guilt was hitting MM pretty hard here. Randall doesn’t believe he is a good guy. He thrives on how evil he is and likes spreading that suffering around. I don’t think MM would have any qualms about pounding him flat