He needed to amp himself to fight the Ashura Avatar. He wasn't struggling prior to that.
In their fight after defeating the final villain, Black Zetsu(not joking, chapter 690), Naruto was absolutely rocking shit.
-Sasuke captured the Bijuu immediately.
-Base-to-Base, they evenly clashed.
-SPSM Naruto blocked his Fireball with a chakra hand, and Sasuke hit him with a Chidori, but that only grazed him, sending him flying.
-Kurama Avatar blocked Perfect Susano'o's sword slash with 1 tail, then they evenly clashed with punches.
-Sasuke said "You're just playing defense?" and Naruto went "I don't want to kill you."
-A regular Bijuu bomb was equal to Susano'o Chidori, and then Naruto said he wasn't trying to kill Sasuke again.
-Sasuke used Indra Susano'o and Naruto used 3 Kurama Avatars, who were, quite frankly, holding their own before Sasuke pulled out the arrow.
-They both use their best forms, their best attacks, clash, and revert to base.
-Naruto uses Kurama Chakra to counter Amaterasu.
-4 Shadow Clones start piecing up Sasuke. He takes out 3 with Chidori, and Naruto floors him with a punch.
-Naruto nearly hits him with a Rasengan, trips, and Sasuke starts hitting him while he's down.
-Naruto floors him with one headbutt.
-They keep punching each other, and are still fairly even base-to-base.
-Sasuke absorbs some Kurama chakra from Naruto, goes for a Chidori, and receives a RIDICULOUS uppercut, hitting the rock wall so hard it shatters.
-Sasuke infuses Amaterasu into his Chidori, and Naruto uses a regular Rasengan, and that's still only an even clash.
Naruto did not have full Kurama for this whole fight.