Under these conditions, I don't know who wins so I'll try thinking up of a scenario.
As the fight starts, Kakuzu releases his masks while Mei uses her water wall, creates two Mizu bunshins, transforms that water wall into a water dragon which Kakuzu tanks with doton domu. By this time Kakuzu will have released his masks and the Mizu bunshins will have gone underground which Kakuzu won't notice due to being hidden by Mei's water wall. By this time, Kakuzu will send his katon + fuuton masks for a frontal assault which is further backed by an assault from the suiton and raiton masks which attack Mei from a different direction. Mei uses mobile Earth core to avoid getting hit by the four elemental attacks and to gain some high ground over the flying masks and Kakuzu. Right afterwards, Mei's suiton bunshin blindside Kakuzu from his blind spot, one uses a suiton and the other uses lava to harden Kakuzu. Kakuzu survives due to doton domu but his body is now hardened. Luckily for Kakuzu, the two Mizu bunshins run out of chakra after their attack and disperse.
The masks then divert their attention to defending Kakuzu allowing Mei to snipe one of them with Suidanha. Kakuzu eventually uses his domu enhanced strength to break free along with his fuuton mask to destroy the concrete. Knowing Mei can easily blindside him, from now on Kakuzu decides to keep a mask at his blind spot at all times. With one mask destroyed, the doton mask inside Kakuzu and one mask guarding Kakuzu's blindspot, Kakuzu only has two hearts to attack Mei offensively.
In an attempt to overwhelm Mei, Kakuzu will decide to use his threads to attack in conjunction with his two offensive masks. Mei knows what is going on and realizes that the only way to win is by splitting up the masks and Kakuzu from each other so she uses hiding like a mole to escape a fair distance away and then uses acid mist to conceal her location and do damage causing Kakuzu to panic. Kakuzu will end up fusing with his masks to prevent his masks from getting blindsided and because they are safe inside his body and uses his katon and/or fuuton masks to clear out the mist. Regardless, the noise created by Kakuzu's jutsu coupled with all the smoke/debris from Kakuzu's own jutsu allows Mei to hit Kakuzu with lava which he can't evade and ends up utilizing doton domu. Regardless of him activating doton domu, the masks on his shoulders get killed by the lava. With lava on his body, Kakuzu ends up using the threads on his body to remove the lava and attack Mei with those lava soaked threads but during that time Mei manages to fire even more lava at Kakuzu. Due to being close to Kakuzu and having exhausted herself, Mei can't dodge Kakuzu's attack and gets impaled by the lava threads thus she dies but the lava on Kakuzu's body ends up seeping through, killing the masks inside his body and melting his body.
So under these conditions, I think a draw is likely.
