The Gray Mist Fiend wasn't impossible to kill. Rather, its true body was a dense gray mist. This mist gathered and took the form of ghostly figures.
Because of this,
although Xu Qing was powerful enough to easily disperse the ghostly figures formed by the gray mist, the mist itself was not significantly damaged when he did so.
So,
the gray mist could simply reform.
This created the illusion that it was unkillable.
After Xu Qing summoned the Witch Evil Ghost, it didn't just disperse the ghostly figures. It tore them apart and devoured them.
With the gray mist being devoured by the Witch Evil Ghost, it suffered a devastating blow. Once all the ghostly figures were eaten, the 'Gray Mist Fiend' was naturally eliminated.
Around him,
Xu Qing clearly saw all the gray mist dissipate, and his vision returned to normal. The number of Eerie Walking Corpses around him was growing, and they quickly closed in.
"Thirteenth Sword! Heaven-Filling Sword Shadows!"
CLANG!
