那曾用剝 No Stripping Required
行者放下道：「此乃妖精穿的衣服。」八戒道：「怎麼就有這許多？」行者道：「七套。」八戒道：「如何剝得這般容易，又剝得乾淨？」行者道：「那曾用剝。行者放下道：「此乃妖精穿的衣服。」八戒道：「怎么就有这许多？」行者道：「七套。」八戒道：「如何剥得这般容易，又剥得干净？」行者道：「那曾用剥。
逐句解 Line by line
Pilgrim put them down and said, "These are clothes the monsters were wearing."
此乃 is bookish — these are — where the speech around it is colloquial. Pilgrim slides between registers constantly and it is one of the ways the book marks him as older than he behaves.
Eight Rules said, "How is it there are so many?"
這許多 — this many, such a lot. Ten characters, and he is counting rather than thinking.
Pilgrim said, "Seven suits."
Five characters, one of the shortest speeches in the chapter. He does not elaborate because the number is about to do the work.
Eight Rules said, "How did you get them stripped off so easily, and so cleanly?"
得 is de both times, joining the stripping to how it went — not dé, to obtain. 乾 is gān in 乾淨, clean: the other reading qián belongs to the trigram and to 乾坤, heaven and earth. Same character, entirely different word.
Pilgrim said, "There was no stripping about it."
那 is nà — as if, where would there have been. Seven characters, and it is Pilgrim correcting the assumption behind the question rather than answering it.
Eight Rules asks how the clothes came off so easily and so cleanly, and the question tells you what he has assumed happened.
Pilgrim's answer is a flat correction: there was no stripping. He took them off a rack. The two of them are having slightly different conversations, and Eight Rules will keep having his for the rest of the chapter.
One small point of writing. The character for clean here is the same one that means the first trigram of the Book of Changes, heaven itself, in a completely unrelated word. Nothing in the writing distinguishes them; only the company does.