心焦口渴 Heart Scorched And Mouth Dry
且不說他兩個相鬥難分。卻表唐僧坐在途中，一則火氣蒸人，二來心焦口渴，對火焰山土地道：「敢問尊神，那牛魔王法力如何？」土地道：「那牛王神通不小，法力無邊，正是孫大聖的敵手。」且不说他两个相斗难分。却表唐僧坐在途中，一则火气蒸人，二来心焦口渴，对火焰山土地道：「敢问尊神，那牛魔王法力如何？」土地道：「那牛王神通不小，法力无边，正是孙大圣的敌手。」
逐句解 Line by line
But leave the two of them locked past parting.
且不說 is the storyteller's turn-away, closing one thread to open another. The poem has just ended and the narrative changes place in a single line. 相 is xiāng, "each other"; 難 is nán and 分 is fēn, so 不 stays bù.
The tale turns instead to the Tang Monk, sitting on the road. For one thing the fire-vapour steamed him, and for another his heart was scorched and his mouth dry; and he said to the local god of the Fiery Mountain: "May I ask, honoured spirit, what are the Bull Demon King's powers?"
From thirteen lines of aerial combat to a hot man sitting on a road. 一則 is the counting word "for one thing", so 一 holds at yī. This is the one place in the chapter so far where the ordinary tone-shift rule has to be overridden rather than merely confirmed — the rule would lower 一 to yì before the rising tone of 則, but the character is enumerating rather than counting, and the 二來 that follows makes the enumeration explicit. 途中 takes 中 at zhōng.
The local god said: "That Bull King's arts are not small and his power has no bound; he is a match for the Great Sage exactly."
敵手 is an opponent of equal standing rather than merely an enemy, and the local god is answering the question that was actually asked. (不 shifts to bù before the dipping tone of 小.) The judgement is the narrative's own: the poem has just spent thirteen lines demonstrating it.
The turn away from the fight is abrupt, and the contrast is deliberate. Thirteen lines of aerial combat give way to a man sitting on a road in the heat, and the chapter's two registers are set side by side without a word of comment.
一則火氣蒸人，二來心焦口渴 lists Tripitaka's condition in the flattest form available, and the flatness is the point. He is not in danger, only uncomfortable — and his discomfort is what is about to send two disciples into the fight. The entire machinery of this episode rests on a hot and thirsty man.
The local god's answer is worth weighing. 正是孫大聖的敵手 — an exact match for the Great Sage — and the reader has just watched thirteen lines proving it. This is not reassurance offered to a worried master. It is a straight assessment, and it makes the worry reasonable.