一時興發 The Idiot Gets His Blood Up
那獃子一時興發，掣了釘鈀，叫聲：「去來。」他兩個不顧師父，一齊駕風趕上，舉釘鈀，使寶杖，望妖精亂打。那妖精戰行者一個已是不能，又見他二人，怎生抵敵，急回頭，抽身就走。那呆子一时兴发，掣了钉钯，叫声：「去来。」他两个不顾师父，一齐驾风赶上，举钉钯，使宝杖，望妖精乱打。那妖精战行者一个已是不能，又见他二人，怎生抵敌，急回头，抽身就走。
逐句解 Line by line
The Idiot's blood was up in a moment. He hauled out his muckrake and shouted, "Let us go!"
一時興發 is a fit of enthusiasm coming over someone all at once, and 興 is the mood-and-appetite reading, xìng, not the xīng of setting a thing going. Bajie said no, no, no a passage ago; a proverb about wind has changed his mind completely. 釘鈀 is his nine-toothed rake, and it is a farm tool — the weapon fits the man.
The pair of them left the master where he was, rode the wind up together, and laid into the fiend, the rake swinging and the treasure staff coming down.
不顧師父 — they paid the master no mind. Four characters, said flatly, and the whole disaster of the next hour is inside them. The novel does not underline it; it just puts it in the sentence and moves on.
The fiend had been unable to beat Pilgrim single-handed. Now that she saw the other two as well, how was she to hold out? She whipped her head round and turned to run.
怎生 is how on earth, a set phrase for a question with no good answer, and it is one of the novel's favourite ways of putting the reader briefly inside a character's head. 抽身 is to pull oneself free and go, still the ordinary word for extracting yourself from a situation.
Bajie's reversal is instant and unembarrassed, which is exactly why he is funny. He is not a coward who becomes brave; he is a man whose opinion is whatever was said to him most recently.
The fatal detail is the flattest one in the passage. They left the master where he was. Everything that goes wrong for the rest of the chapter follows from that half-line, and the novel refuses to make a fuss of it.