So what?
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How does a sand coffin has penetrating abilities comparable to chidori when sand coffin is a pressure based attack, not a piercing attack? Chidori can cut through a diamond hard defence such as Kakuzu's with ease. Anyway black chidori is even stronger than regular chidori and will deal a huge amount of damage if it doesn't outright kill him.
Lol if Kimimaro had trouble against Gaara alone, how does he beat all three of them at the same time? The only problem was that Gaara had no way of hurting him but Naruto and Sasuke do.