Mei wins this with high difficulty, unless we make this a close range fight from the start. Mei's hand seal and jutsu execution speed are both very fast[
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]-[
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], so, she can put up Kirigakure no Jutsu/Futton double layer in no time. Tsunade will have to activate Creation Rebirth and/or Byakugō to keep healing from the corrosive mist's effects, which was melting both Madara and Sasuke's ribcage Susano'o, and damaging the latter's skin despite him being covered in said susano'o. Katsuyu will probably be summoned to find Mei within the hidden mist's large AoE, which a weaker Kiri nin's version enveloped
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. Katsuyu would probably disperse into smaller slugs like she did against Manda in order to track Mei, but they can be blown back by Suiton: Suijinchū, which overwhelmed Madara's katon with no difficulty. However, Tsunade will probably get a general idea of Mei's location from the water's direction, which will lead her to blitz said area and punch the ground. Mei can anticipate this though, and move to another spot and wait to ambush and douse her with Yōton before Tsunade can reach her, with Mei having more intel on Tsunade due to her fame as a combat-medic and one of the Sannin. I say high difficulty because there's a chance that Mei will take damage from the shockwaves of Tsunade's ground punches.