裡應外合 Inside Answering Outside
若師父果在裡面，我將鐵棒把妖精從裡打出，跑至門口，你兩個卻在外面擋住：這是裡應外合。打死精靈，才救得師父。」二人遵命。若师父果在里面，我将铁棒把妖精从里打出，跑至门口，你两个却在外面挡住：这是里应外合。打死精灵，才救得师父。」二人遵命。
逐句解 Line by line
"If the master really is inside, I will take my iron rod and drive the fiend out from within, and when she runs for the entrance you two block her on the outside — this is answering within and closing without."
The last four characters are a real military idiom, still in use: 裡應外合, an inside party answering an outside party, the classic arrangement for taking a walled city with a traitor at the gate. In it 應 is yìng, to answer or respond, not the yīng of ought. The chapter's closing couplet will use the same phrase about Pilgrim inside the fiend's stomach, so hold on to it.
"Kill the sprite, and only then can the master be saved."
Here 得 is dé, the full reading — this is the fused ending that makes a verb able to take its object, not the light linking syllable. 精靈 is a sprite or an imp, a slightly diminishing word for her.
The two of them received the order.
Four characters, flat and military. 遵命 is to obey a command, and the narrator uses it to end the planning and start the descent.
The plan is a siege plan, and the idiom Pilgrim reaches for says so. Answering within and closing without describes the oldest trick in Chinese military writing: get one man inside the walls and the walls stop mattering.
Keep the phrase. This chapter ends on a couplet that reuses it, and by then the inside will not be a cave but a body — Pilgrim answering from within the fiend's stomach while Bajie and Sha Monk hold the door. The novel plants the vocabulary here and collects on it in the chapter's very last line.
The four-character obedience at the end is doing structural work. It closes the conversation the way a stage direction closes a scene, and the next passage begins in mid-air.