So...
It was not that Water Ghost could not defeat Jiang Buyi at the Lone Cliff.
However, in a one-on-one fight, he would have to undergo sanctification or pay a huge price to take down the other party. His identity as Yu Mo would also be exposed.
In the end, he only killed one person and obtained a semi-saint person.
That was all!
But to introduce it into the chess game...
He had to take advantage of the timing: the approach of the big force, the chaos and the arrival of the vortex;
Use the geographical advantage: the Spell Forbidden Barrier in the deep sea, Abyss Island's banishment rules that forced half the demi-saints out of existence.
Take advantage of people: Use Yama's hands to kill Jiang Buyi, use Jiang Buyi to stir up chaos in the Holy Divine Palace, use the distraction of the attention of the people of the Holy Divine Palace to secretly complete his ultimate goal.