Good fight but Kakuzu would take it. They both can cancel eachother techs because they both know alot of elements(idk if mei knows lightning or wind) Kakuzu would win because he just has more up his sleeve and harder to kills.
Just a little side note: To melt diamond requires pressures (synthesized by magnetic fields) over 10 million times stronger than the pressure the atmosphere exerts at sea level. Lava would not come anywhere near this