Don't forget Katsuyu. She can tank Tsunade too plus she got her acid for long range attack.If Mei keeps her distance, she can defeat Tsunade. Mei can keep spamming her long range acid/lava attack until Tsunade can't use the regenerative abilities of the Byakugo Seal anymore when her chakra is depleted from it. Then she will melt/corrode to death. Tsunade can only win if she manages to close in on Mei.