斬草除根 Cut The Weeds Pull The Roots
行者道：「憑你如何主張？」八戒道：「依我，先打殺了妖精，再去解放師父：此乃斬草除根之計。」行者道：「我是不打他，你要打，你去打他。」行者道：「凭你如何主张？」八戒道：「依我，先打杀了妖精，再去解放师父：此乃斩草除根之计。」行者道：「我是不打他，你要打，你去打他。」
逐句解 Line by line
Pilgrim said, "Then what do you propose?"
憑你 — as you please, up to you. Nine characters, and it is a capitulation dressed as a question. He has lost the argument and is handing over.
Eight Rules said, "If it were up to me: kill the monsters first, then go and release the master. That is what they call cutting the weeds and pulling out the roots."
解 is jiě, to loose or untie. 斬草除根 is a real four-character saying, still in daily use, and it is the exact opposite of the 留根 he opened with — leave the root in, or take the root out. He built the whole argument to land on it.
Pilgrim said, "I am not going to hit them. If you want to hit them, you go and hit them."
Fifteen characters, and the pronoun shifts three times: I will not, you want to, you go. He concedes the argument and refuses the job in the same breath, which is a manoeuvre anybody who has worked in an office will recognise.
Eight Rules wins the argument and lands it on the phrase he has been building toward since he opened his mouth: cut the weeds and pull out the roots. It is still an everyday expression in Chinese, and it means finish the job so it cannot come back.
The pair of images is the whole speech in miniature. He began by accusing his brother of leaving the root in and ends by naming the remedy. For a character the narrator calls a blockhead, it is a well-made argument.
And Pilgrim's answer is the funniest and least admirable line he gets in the chapter. He does not defend his position. He says he will not do it, and that if Eight Rules wants it done, Eight Rules can go and do it. Having lost on the merits, he delegates.