I think Superman will rip Monarchs armor apart or damage it, which will release the energies. He needed to leave the double Big Bang of Imperiex Prime in the past or he would get killed IIRC. Since he has no Boom Tube I think he would die right after Monarch explodes.
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Anyway, I think that OWAW Supes would win without the amp but as said, it would cost him his life as he has no means to contain Monarchs energies.
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I think if Superboy Prime could [whose amp had wore off at that point], he could, too. I think all-out Superman [OWAW] is Superboy Prime level - nevermind with the amp.
I wouldn't lump Imperiex and Monarch in the same boat, though. Imperiex is basically entropy/creation/omni-dimensional annihilator. He is the big bang, in the literal sense - from which every dimension of the Universe is reborn. Monarch is quantum-destruction.
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