There is an easier way with Itachi's first form, which brings up another quicktime event. When Itachi throws his charged up shurikens, throw charged up shurikens of your own at the same time. This will bring up the quicktime event where you have to tap x faster than he does, and deals significant damage. Do this about four or five times and his first form is down. DO NOT use items at this point - they will be important in the next part. This is, in my opinion, the hardest battle for quicktime events in the game (and therefore to S rank), so good luck and be good at tapping vigorously. This took a good few times and I only S ranked it by memorising the sequence.
The fight against Susano'o is best taken by hitting A twice, therefore entering your dodge tactic. Dodge all moves and then when the arm comes down to protect Itachi from the front, go in at the side and mash B. After a while it will come up with a Y above his head. Hit it now for more significant damage. You have to be quick though because otherwise the Susano'o will recover and deal you a hefty whack. Go back into dodge after hitting Y and repeat. Use items when you deem necessary to increase your attack or defence - these are the most valuable items in the game and I would certainly recommend them for this fight.
The last step of the fight seems strange the first time you do it, but all you have to do is keep stepping back. Do not even attempt to fight Itachi. Cue cutscene.