This is just the way people can't accept how powerful Itachi is. Orochimaru had even more knowledge about the Sharingan, he attacked Itachi for the purpose of obtaining his Sharingan but failed. Itachi can ue Genjutsus with any part of his body, he also can equip his crows with a Sharingan and send them to Mei, in order to catch her if she's looking into the crows eyes. It's not just Itachis Genjutsu, but also his great analytical skills and intelligence, which allows him to read the opponent like an open book. Kagelevel doesn't mean anything, Itachi was Anbu leader at 13 and could become Hokage as well, he did one shot a potential candidate for the Hokages throne with a mere Genjutsu, when he was just a child. It's not enough to avoid eye contact (what already is hard, as looking into the opponents eyes is a matter of habit), Kabuto for example had to close his eyes completely. Even if Mei will look only at Itachis feet, latter however could easily provoke eye contact. She simply can't fight someone of Itachis level while looking at his feet, considering his speed and feints. Genjusu, Karasubunshin and explosive Bunshin, his speed not even the Sharingan can properly keep up with, his analytical skills allowing hiim to read the opponent like an open book and various other skills, he was even praised among his talented clan. It's an obviousness that Itachi doesn't need to put in effort for fighting someone like Mei, who has nothing but notable Ninjutsu.