Ok let me explain:
Pain get's one shotted. Why? Because it's Manga intel...Pain doesn't know about Gai's ability meanwhile, we've seen Gai knows about Pains ability
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Therefore if Gai uses Hirudora right off the bat(Since he has knowledge), it would be just Kisame's case
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One of the Pains would jump to absorb it like here
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and they all get killed due to the range it covers.
I can create another scenario for the Jounins if both Parties had full intel.