教他斷根 Cut Them Off at the Root
行者道：「且住，洞裡還有小妖怪未拿哩。」八戒道：「哥啊，有便帶我進去打他。」行者道：「打又費工夫了，不若尋些柴，教他斷根罷。」行者道：「且住，洞里还有小妖怪未拿哩。」八戒道：「哥啊，有便带我进去打他。」行者道：「打又费工夫了，不若寻些柴，教他断根罢。」
逐句解 Line by line
Pilgrim said, "Hold off. There are still little fiends in the cave not yet accounted for."
未拿 is not yet taken. The flourish 哩 closes the line with that shrug again. Wukong is not sparing the old fiend; he is keeping him alive until the rest of the job is done.
Bajie said, "Brother, if there are, take me in with you and let me beat them."
Thirteen characters, and Bajie's enthusiasm for going back into a cave he was afraid of two days ago is entirely a function of everybody inside being asleep.
Pilgrim said, "Beating them would only cost us the trouble. Better to find some firewood and cut them off at the root."
費工夫 is to cost time and effort, and 夫 takes an unstressed ending in it. 斷根, to cut off at the root, is a gardener's phrase used for wiping out a line so nothing grows back. And 罷 is ba here, the softening final word — which puts a mild, agreeable little tag on a proposal to burn a household alive.
The reasoning is efficiency, stated plainly. Fighting a hundred sleeping fiends one at a time would take all afternoon; fire takes one match.
The softening word at the end of that sentence is the most disturbing thing in the passage. It is the tag you put on a suggestion about where to have lunch, and Wukong attaches it to a proposal for arson.