Start a battle and select Karin at the 'Character' selection. Then choose Taka Sasuke (Akatsuki Sasuke) and wait for the time limit to expire. The game will then automatically select Sasuke Chidori True Spear as a playable character.
When you have completed the game return to Mount Myoboku and go up the stairs to the house. Pray at the statue inside to unlock the alternate costume. You must have the Hokage Naruto costume to get the 'All Ninja Assemble' Trophy.