After beating the main storyline, do a few request events until you get a letter from bird addressed to Izumo in the Hidden Leaf Village. Complete that quest to unlock the use of messenger birds. Do a few more request events until you get another letter while in the Hidden Leaf Village telling you that Killer Bee is waiting for you at the main gate. Defeat the imposter Killer Bee. This starts the "White Zetsu Subjugation" special quest. Defeat the imposters given to you through wanted lists until you get to the final mob battle against the White Zetsus with Nine-Tailed Chakra Naruto. After completing the "White Zetsu Subjugation" special quest, you unlock Survival Mode for Free Battle.