Umm, Itachi's Amaterasu would only work on ONE point on Sasori's body. If he misses, Sasori can switch. And either way, Sasori knows his weak spot, and against a strong opponent like Itachi, he would certainly defend it better than he did against Chiyo and Sakura. Even a small barrier would defend him enough.
Otherwise, Sasori's 100 puppets would certainly provide him the opportunity of victory, as well as the iron sand he controls. He can fly using iron sand, and go out of reach of Itachi's blade if he so wishes. Sasori would also FORCE Itachi to keep up Susanoo, which in fact could a trap for poison gas, because the mobility of Susanoo is minimal. But without Susanoo, Itachi gets overwhelmed. Genjutsu is actually Itachi's only chance, and we still cannot be sure that it would work on Sasori.