Kakuzu would win in the end due to his overwhelming power in ninjutsu and his threads but it's not easy as some people believe it to be, Mei can easily surround a large portion of area around her with boil/acid mist, if the masks were to come into contact with the mist I dont see them coming out of it unharmed, they likely get destroyed within seconds once they touch the best. And their's hidden mist no jutsu, I only give this to kakuzu because of his speed and experience against hashirama and domu, his threads wont even be a factor once they come into contact with Mei's boil/acid mist, they get incinerated upon contact.