I love Mei a lot, heck if I was one of the naruto characters I would probably have a crush on her, she's real strong too, maybe not physically, but her elemental ninjutsu prowess is unmatched in the narutoverse, she has MASTERED earth, water, and fire affinities and has two powerfull sub elements of lava and boil release, definately dominating the elemental ninjutsu department, even better than kakuzu probably.
But that is the main problem isn't it, anyone whose main offense is based almost solely on ninjutsu is almost, if not gauranteed to lose against kisame. He has samehada to absorb all the chakra out of you justu and even if you somehow disposess him of the sword, he still has that great shark jutsu that is unstopable because it keeps getting bigger and stronger by absorbing chakra from anything it comes into contact with, making it impossible to stob with a chakra based jutsu, which is what all jutsu are off course, except for Guy's morning peacock and afternoon tiger techs, which are just super taijutsu.
Mei will lose definately here.