Physical strength? Tsunade no contest
Strength as a kunochi? Mei U_U
Tsunade is not going through a ton of lava plus acidic mist which can both be used as a defense without wasting her chakra whiles Mei can continue to send a barrage of attacks from long range U_U Tsunade's chakra will eventually run out having to continuously heal while dodging Mei's acid mist aswell as having to move through a terrain of lava