這等猖狂 Letting Her Rage Like This
怎麼又從他口裡出來，卻與他爭戰，讓他這等猖狂？」沙僧道：「正是。卻也虧了師兄深洞中救出師父，返又與妖精廝戰。怎么又从他口里出来，却与他争战，让他这等猖狂？」沙僧道：「正是。却也亏了师兄深洞中救出师父，返又与妖精厮战。
逐句解 Line by line
Why come out through her mouth instead, and stand there trading blows, and let her rage about like this?"
猖狂 is unrestrained, running wild, and it has kept that force in modern Chinese — it is what you call an enemy who is having far too good a time. Bajie is not worried; he is offended.
Sha Monk said, "Just so.
Sha Monk agreeing in two syllables is characterisation by economy. He does not contradict Bajie, and he does not join in the complaining either — he simply adds what Bajie has left out.
Still, we have our elder brother to thank for getting the master out of the deep cave, and now he is fighting the fiend on top of that.
中 is zhōng, inside — the deep cave. The other reading, zhòng, means to hit a target or to be fit for something, and the two are kept strictly apart. 虧了 is thanks to, and it still carries the faint sense of a debt owed; 廝戰 is to grapple, an old word for fighting at close quarters.
Sha Monk's function in these scenes is to be the one who keeps score honestly. He agrees with the grievance and then quietly lists what Pilgrim has actually achieved, which is more than either of them has.
The disciples spend this passage misjudging a rescue in progress. That misjudgement is about to become an action, and the action is about to cost them the master.