Ay is like a fast tsunade and his speed is faaaaaaaaar too much for her. Mei wellI explained that. lariat>tsunade
That's not how a ranking list works, at least not for me.
Just because i ranked her higher doesn't mean other characters placed lower can't beat her. If i keep trying to figure out who's > than who i'll never finish a ranking list.
The only thing Ay has on Tsunade is speed and one single elemental nature (which he only casts on himself and doesn't wield with much proficiency outside that). Overall she brings way much more to the table, that's why i put her above him. The only reason why i didn't place him last is because he can (in normal conditions) pressure an opponent much harder than Mei, he's achieved more and i've been more impressed with him.
I placed Mei last because while her offense is solid and she has lots of versatility, she doesn't seem to be as well rounded as her arsenal implies nor as effective.