燥癢難禁 An Itch He Cannot Bear
那些毫毛即變做三樣惡物，乃虱子、虼蚤、臭蟲，攻入妖王身內，挨著皮膚亂咬。那妖王燥癢難禁，伸手入懷揣摸揉癢，用指頭捏出幾個虱子來，拿近燈前觀看。那些毫毛即变做三样恶物，乃虱子、虼蚤、臭虫，攻入妖王身内，挨着皮肤乱咬。那妖王燥痒难禁，伸手入怀揣摸揉痒，用指头捏出几个虱子来，拿近灯前观看。
逐句解 Line by line
The hairs turned at once into three nasty creatures — lice, fleas and bedbugs —
The great sage of the western pilgrimage deploys body lice. 子 in 虱子 is the neutral noun ending zi, not the full syllable of a philosopher's title.
which drove in under the fiend king's clothes and set about biting all over his skin.
攻 is a military word — to assault, to storm — used here of fleas. 著 is zhe, the durative: they stay against the skin while they bite.
The fiend king, itching past all bearing, put a hand inside his robe to feel about and rub at it,
禁 is jīn, not the reading for forbidding — 康熙 示部八畫 gives the endurance sense its own band at 居吟切, 音金, glossed 力所勝也，當也, what one's strength can bear. 揣 is chuǎi: the dictionary opens at 初委切 with 說文 量也 and then 一曰捫而察之也 — to feel and examine by hand, which is exactly what he is doing.
pinched out several lice with his fingers, and held them up to the lamp to look.
A thoroughly undignified sentence, and it is written perfectly straight. 觀看 doubles two verbs of looking, which makes the inspection careful and therefore worse.
Nothing about this attack is heroic and that is the whole idea. Wukong could fight — he has already tried, in this chapter, and lost. So instead he chews up a handful of his own hair and turns it into lice, fleas and bedbugs, and sends them under the enemy's shirt.
攻入 is a siege word. Armies 攻入 a city. Here it is fleas going in at the collar, and the mismatch between the vocabulary and the vermin is the joke — grand words for a small indignity, which is one of the reasons Wukong is funny rather than merely powerful.
The last image is the one that finishes him. A monster king who has beaten vanguards and killed officers is now holding a louse up to a lamp and squinting at it. The next passage is about his embarrassment, and it deserves to be.