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Mei has attacks like a giant water Dragon that make Kakuzu's biggest attack seem piddling. Mei can do it with one seal too, so she just spams that at Kakuzu who is battered around like a doll hundreds of feet around the battlefield... when an opening presents itself, Mei just spits Lava at him (or uses boil release), both of which should be ample to kill Kakuzu. The scale of Mei's lava blasts alone is sufficient to take out Kakuzu (see the scans on the other thread, her attacks are huge and fast). Nor does Earth spear protect Kakuzu from breathing in corrosive gases and dying.