you talked about kakashi vs kakuzu+ hearts and hidan ,that was not 1on1 .and saved shika team.
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and
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the only part where kakashi got difficulty against kakuzu was in the end of fight.but kakashi's strategy was to gain time for shika give blood to hidan.
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kakashi didn't have to waste chakra for use kamui against kakuzu but if shika would be a bit later ,he could use it and kill kakuzu despite to go hospital

kishi did it to show us that kakashi could kill kakuzu .it's not an hazard.
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