Originally posted by AuraAngel
He lost to Naruto while Kurama was off doing something else. Naruto in his base form, with the chakra only around his hand, stopped the moon splitting attack and curbed him pretty solidly. Naruto is certainly more powerful working with Kurama than Naruto on his own.
You forget that Naruto only possessed half of Kurama during the War arc, and that he possessed both halves during The LAST.
When Kurama was completely released, he appeared in physical form during The LAST. The fact that he was a chakra avatar, paired with the fact that Naruto still had BSM activated, points toward only a portion of Kurama being diverted to the Golem. Not that it matters; Toneri was also powering the Golem, it's perfectly fair in the end.
Also, that wasn't Naruto in base. That was actually one of the more interesting parts of the fight; he condensed the chakra of BSM into his hand, which allowed him to stop the Moon-Cutter and defeat Toneri with a Falcon Punch.
Toneri is a powerful Otsutsuki that possesses Rinnegan-calibre dojutsu. I'm unconvinced that Naruto during the War arc could have handled him, honestly.