協力同心 Send Them Both With You
長老道：「寡不敵眾，是你一人也難處。八戒、沙僧他也都有本事，教他們都去，與你協力同心，掃淨山路，保我過去罷。」行者沉吟道：「師言最當。著沙僧保護你，著八戒跟我去罷。」长老道：「寡不敌众，是你一人也难处。八戒、沙僧他也都有本事，教他们都去，与你协力同心，扫净山路，保我过去罢。」行者沉吟道：「师言最当。着沙僧保护你，着八戒跟我去罢。」
逐句解 Line by line
The elder said, "The few are no match for the many. It is hard going for you alone."
寡不敵眾 is a classical four-character saying, older and more formal than the two Wukong just used. The master answers proverbs with a proverb, and picks a grander one.
"Bajie and Sha Monk both have abilities of their own. Have them both go, join strength and heart with you, sweep the mountain road clean, and get me across."
協力同心, joining strength and sharing one heart, is still current. Xuanzang's solution is managerial and entirely sensible, and it is also precisely what Wukong manoeuvred him into saying.
Wukong pondered and said, "The Master's words are most apt."
當 is dàng here, the falling tone — 康熙 files that band at 丁浪切 and glosses it 事理合宜也, fitting to the reason of things. It is not the dāng of ought or of serving as.
"Have Sha Monk guard you, and have Bajie come along with me."
Both 著 are zhuó — the causative band, have somebody do something, which is a different reading again from the light zhe that has been running through this chapter and from the zháo of the verse couplet. And note the edit: the master said send them both, and Wukong is taking one.
The negotiation closes exactly where Wukong wanted it. He complained about being outnumbered, his master offered him both companions, and he takes the strong one and leaves the reliable one on guard duty. It is a good tactical decision and it is also the answer he had in mind before he started quoting proverbs.
師言最當 is worth hearing for what it is. Wukong praises the master's judgement in the same breath that he amends it. Anyone who has sat in a meeting will recognise the move.