The situation favors Mei immensely but still, Sharingan can see chakra, and I doubt light is necessary for that. She will have to use the hidden mist technique right at the start. If she can do that, she wins. Itachi won't be able to use Amaterasu and Tsukuyomi because of the mist, and Susano'o will delay the inevitable. Itachi would eventually tire and the Acid Mist would melt him. Still, this all depends on Mei putting up Hidden Mist before Itachi decides to use Amaterasu. Even if she put it up before he could do anything, Amaterasu could possibly evaporate the mist.