Amaterasu is powerful. Sasuke just had to secretly, when Kaguya was busy with Naruto, cast it on her. She would have burned to hell.
But why didn't Sasuke use Amaterasu on Kaguya?
Amaterasu is powerful. Sasuke just had to secretly, when Kaguya was busy with Naruto, cast it on her. She would have burned to hell.
But why didn't Sasuke use Amaterasu on Kaguya?
Given she be the first to wield chakra, Kaguya can absorb any technique; including the Amaterasu
As the first person to ever wield chakra, she can absorb any technique that she encounters
However, towards the beginning of Naruto's and Sasuke's battle with Kaguya, Sasuke did use the Amaterasu on Kaguya to distract her
To create an opening, Naruto convinced Sasuke to attack Kaguya with Amaterasu, which allowed him to get close to her and use his Sexy: Reverse Harem Technique. The technique was successful in distracting Kaguya long enough for Naruto to strike her
You can see this occurrence on Youtube
It's because like the Rinnegan, the dojutsu on her forehead, the Rinne-Sharingan, can absorb all forms of ninjutsu. Even if they used Amaterasu after some distracting attack, since she can activate her Rinne-Sharingan and BYAKUGAN, with nearly 360 degrees of vision, it'd make it pointless. Byakugan is shown to have almost no weak/blind spots, making suprise/diversionary attacks completely useless on Byakugan users, unless they have recently awakened it.