The fiend, hearing this, hastily passed the word of command, mustering all at once the host of demons before and behind the mountain and the fiends within and without the cave; each took up his weapon, and they barred the three or four gates thick and fast, letting none pass.

傳號令, chuán hào lìng, to pass down a command; 各執器械, gè zhí qì xiè, each taking up his weapon; 攔阻, lán zǔ, to bar, to block. Three or four gates sealed thick — the demon has decided to make it a siege.

Pilgrim, seeing this, was glad at heart; taking the staff in both hands, he gave a shout: "Change!"

滿心歡喜, "glad at heart," is the reaction of a man who has just been handed exactly what he wanted. 理棍, lǐ gùn, to grip and ready the staff; 喝聲叫, hè shēng jiào, to give a great shout, with 喝 read hè.

— and turned into a thing of three heads and six arms.

三頭六臂, sān tóu liù bì, "three heads and six arms," is Wukong's multiplying battle-form, borrowed from Buddhist iconography — and the phrase is still ordinary Chinese for superhuman capacity: the thing you say you have not got when somebody asks you to do six jobs at once.

Giving the gold-banded staff a shake, he made of it three gold-banded staves.

金箍棒, jīn gū bàng, the gold-banded cudgel, one of the most famous weapons in Chinese fiction. Three of him, three of it. 幌一幌, huǎng yì huǎng, to give it a shake.

See him now, six hands plying three staves, laying about him all along the way — for all the world like a tiger loosed among sheep, a hawk stooping on a hen-coop.

The paired similes are stock and perfectly judged: 虎入羊群, a tiger loosed among sheep, and 鷹來雞柵, a hawk arriving at a hen-coop — 柵, zhà, the coop or pen. Both measure the same thing, which is not a fight but a harvest.

Piteous were the lesser fiends: those his staff grazed had their heads dashed to powder; those it raked, their blood ran like water.

湯著, tāng zháo, struck a glancing blow, grazed; 刮著, guā zháo, scraped, raked. The distinction is between being touched by the staff and being brushed by it, and neither is survivable.