What does Wei Zheng mean by slashing the Dragon King?

This is the ninth wonderful and selfless song of Yuan Shoucheng from The Journey to the West. The Journey to the West Episode 7 Breaking the Black River.

In order to make Yuan Shoucheng's divination fail, the King of Jinghe changed the time and amount of rain without authorization. In this way, he smashed Yuan Shoucheng's stall on this land. 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 pleaded 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 behead the Dragon King.