This is why i think Onoki wins:
In the ten seconds of prep time he's given, onoki can construct a basic Doton structure blocking Itachi from view. Now Itachi cannot one-shot him with Amaterasu or genjutsu. In these ten seconds of prep time, he can also create a bunch of rock clones. Now he can just sit back and let his clones do most of the work. They could use weighted boulder jutsu on his Susano'o to break through it or make it yo heavy to move. Since genjutsu doesn't work on clones, Itachi would be forced to use amaterasu on them and thus waste his chakra. While his clones are pressuring him, Onoki can either prepare a massive Jinton from a safe distance behind his rock encampment and then delete Itachi along with his Susano'o, OR he can use his rock golem to go underground and snatch Itachi out from under his Susano'o.