The video above shows the Naruto vs. Sasuke Final Boss Fight and the Game Ending.

Completing the last chapter with S Rank unlocks “The Last Man Standing” trophy / achievement. S Ranks are all based on how much health you have left at the end of the battle and how many stars you get during QTEs. You must have over 90% of the first health bar left and get all QTE stars for the S Rank. There are two fighting sections that count for this (3:58-5:17 & 15:05-22:00). The rest is just cutscenes and the game ending.
Best way to get S Rank is to use chakra dash and then perform a combo. Press L2 (PS4) / LT (XB1) to evade enemy attacks instantly. During QTEs just smash all buttons very quickly and you’ll easily get 100% of the stars. You don’t have to finish both fights in a row with S rank. They can be selected individually from the story board.