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Zoro can damage Kaido, by using the scar that Oden created and the scar that scabbards damaged a little. Zoro knows his role, he will try to find a cheap-shot, while others are distracting Kaido. Luffy will still show the level difference between himself and Zoro.
Kaido is a special case because they need a swordsman to open up the wound that swordsman Oden caused before and the scabbards already damaged that scar a little, that's also why Oda used scabbards to fight with Kaido, otherwise he wouldn't do it, he also gave Oden's sword to Zoro for that reason. These are special circumstances that wouldn't happen with Luffy's other opponents, and again Luffy will still show the power difference between himself and Zoro despite Zoro opening the old wound that Oden caused and the scabbards aimed and also caused a little damage before Zoro finally opening up again. Kaido already ready for that after seeing Oden's images on his head, once he sees Oden's sword on Zoro's hands, he is already mentally not good when it comes to Oden, and he will again see Oden's images in his head when he sees Enma. So this is a special case.
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