The black Zetsu is fighting with Mizukage who
guards the Feudal Lords, though the objective of
Black Zetsu attacking the Feudal Lords is to stop
the chance of Mizukage joining the frontlines( a
guerrilla tactic of Tobi). It was shown Chapter
559 that Naruto reached the battle zone of
Mizukage vs black zetsu but overexerted thus
making a fool of himself again.