<p>Roshi would win, and for various reasons.
a) Lava Release is made up of Fire and Earth Release. <small>(Pretty sure most of you knew that already)</small> While Gengetsu's Water <i>could</i> cool down Lava, the substance would still remain and could be ignited yet again, leading for surprise attacks and keeping Gengetsu on his toes throughout the battlefield.
b) Genjutsu won't work against Roshi, due to him being jinchūriki.
c) Gengetsu's water is made-up of <i>oil</i>. Oil heats up, fast. Using basic chemistry, it would be logical to say that Gengetsu's water would probably evaporate, and quite quickly, if it would come in contact with very high temperatures. Even if it would not evaporate, it would produce steam. Gengetsu's
Steaming Danger Tyranny does not <i>control</i> steam <small>(In contrary to what most people believe)</small>, but uses the principal of the water cycle to create high-pressurised explosions.
d) Gengetsu's main arsenal is composed of mid-to-long-range ninjutsu (Hozuki Clan Techniques), while Roshi, as evident by the
Lava Release Chakra Mode, is adept at close-range martial arts. While yes, Gengetsu <i>could</i> keep his distance, his stamina is not great enough for him to produce lots of high-level jutsu quickly, as evident during his fight against Gaara, so the Mizukage would be at a disadvantage. <small>(I am aware that the Second Mizukage was also shown to be somewhat good in Taijutsu, but I'd credit that to Edo Tensei)</small>
e) Roshi in his Tailed Beast Form, assuming he could control it, would punch holes in Gengetsu's strategies and would easily be able to subdue his multiple explosions ninjutsu.
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