What does Wei Zhengmeng mean by cutting the dragon king across the river?

This is The Journey to the West's ninth time, Yuan Shoucheng's ingenious and selfless Song, and the old dragon king's clumsy plan broke the sky. The Journey to the West Episode 7 Breaking the Black Water River

In order to make Yuan Shoucheng's divination fail, King Jinghe privately changed the time and amount of rain. In this way, he used this as a reason to smash Yuan Shoucheng's stall. Yuan Shoucheng thought that the Dragon King would be beheaded by Cao if he disobeyed the imperial edict, and told him that if he wanted to live, he could plead with Emperor Taizong. Emperor Taizong dreamed that the Dragon King came to intercede with him, so he promised him and ordered Wei Zhi to follow him into the palace, so that Wei Zhi would have no time to chop down the Dragon King. During this period, Wei Zhi fell asleep at his desk, and Emperor Taizong could not bear to disturb him. Wei Zhi woke up and executed the Dragon King in his dream.

I hope it helps you.