Except we know none of Hidan's attacks are landing on Itachi.
Even 3T Kakashi was casually parrying his strikes in close quarters.
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Itachi does it easily.
Not to mention Itachi was keeping up with SM Kabuto,even cutting off his horn with a simple clone feint \, keep in mind this was the same Kabuto who dodged Sasuke's arrow.
Meanwhile, Itachi can easily hit him with explosive tags, and over time enough kunai would probably damage him to some extent.
What NarutoX28 mentioned works as well.
His clone was capable of restraining a partially transformed CS2 Sasuke.
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Leaving Hidan open for Itachi to strike a vital point or possibly decapitate him.