倒馬毒樁 The Horse-Felling Poison Stake
三個戰鬥多時，不分勝負。那女怪將身一縱，使出個倒馬毒樁，不覺的把大聖頭皮上扎了一下。行者叫聲：「苦啊！」忍耐不得，負痛敗陣而走。八戒見事不諧，拖著鈀撤身而退。那怪得了勝，收了鋼叉。三个战斗多时，不分胜负。那女怪将身一纵，使出个倒马毒桩，不觉的把大圣头皮上扎了一下。行者叫声：「苦啊！」忍耐不得，负痛败阵而走。八戒见事不谐，拖着钯撤身而退。那怪得了胜，收了钢叉。
逐句解 Line by line
The three of them fought a long while, and there was no telling winner from loser.
不分勝負 is the fixed formula for a drawn fight — you cannot separate the winner from the loser. 分 is fēn, and the 不 stays bù before its first tone.
Then the woman fiend gave her body a spring, brought out her horse-felling poison stake, and before he was aware of it had jabbed the Great Sage once on the scalp.
倒馬毒樁 takes 倒 as dǎo, to bring down or fell; the dào reading means "on the contrary" and would be wrong. A 樁 is a stake driven into the ground, and 倒馬 belongs to the vocabulary of techniques for bringing down a mounted man. It is a grand name for what actually happens, which is one prick on the scalp, delivered so fast that the text says 不覺的 — before he knew it.
Pilgrim let out a cry — “Oh, the pain!” — and being unable to endure it he broke off and made away, carrying his hurt with him in defeat.
忍耐不得 is the "can't manage to" construction and takes 得 as the toneless de. The 不 stays bù either way.
Bajie, seeing that matters were going badly, trailed his rake behind him and drew off.
拖著 is the ongoing-action zhe. 見事不諧 — seeing that things are not going harmoniously — is the standard formula for a character who reads a losing position and withdraws.
The fiend, having won the day, put up her steel trident.
得勝 takes the attainment dé, against the toneless de of 忍耐不得 two lines above. Both 了 are the completed-action marker.
The weapon is named before it is described, and the name does the describing: 倒馬毒樁, the horse-felling poison stake. A 樁 is a post driven into the ground, and 倒馬 comes from the vocabulary of techniques meant to bring down a rider. What actually happens is one jab to the top of Pilgrim's head, so quick that the text says 不覺的, before he was aware of it. The grandeur of the name against the smallness of the act is exactly the joke — and it is a joke that stops being funny immediately.
The head that takes the wound is the one the next passage will list as proof against knife, axe, hammer, sword, thunderbolt, fire and forty-nine days in Laozi's furnace. Nothing in the heavens could mark it. A scorpion in a cave on the road west has marked it with a single prick, and that disproportion is the central fact of the chapter.
It is also worth noticing that she wins outright. She is not driven off, she does not flee, she is not tricked; she holds the field, and the last thing the passage records is her putting up her trident at leisure while both brothers run. Clean defeats of this kind are rare in the novel, and when they come they signal that the problem will have to be solved from outside — which here means fetching help down out of the sky.