Itachi would win relatively easily, his genjutsu prowess makes him tower above Mei, as well as being more intelligent, having a more strategic mind and his general jutsu is more than formidable.
how so?Mei's lava release is the slowest jutsu so far in the manga
so shunshin dodges acid mist?He uses shunshin that is superior to Sasukes?
useless against hidden mist jutsuItachi would win relatively easily, his genjutsu prowess makes him tower above Mei