The cudgel named As-You-Will,

The poem opens by squaring the two weapons like duellists: 如意, the cudgel, against 宣花, the axe. Neither fighter is named first. The gear is.

the axe styled Blossom-Boss.

號 is hào here, a style or byname, not háo, "to cry out." 宣花 names the broad-bladed battle-axe of the storyteller's stock repertoire. The weapon is introduced the way a person would be — by the name it goes under.

The two of them, meeting for the first time, knew nothing of each other's depth; axe and cudgel crossed and clashed, left and right.

乍相逢, "meeting for the first time" (乍 is zhà, "suddenly, just now"), so neither can gauge the other. The whole run is kept as one unit here. 不知 takes bù before the level tone of 知.

The one hid marvels of divine craft,

暗藏神妙, "hid marvels of divine craft," set against 大口稱誇, "boasted with a great loud mouth": concealed power against noise. 一個 takes yí before the falling tone of 個.

the other boasted with a great loud mouth.

稱 is chēng, to declare. Notice where the poem puts the boast — in the 大口, the "great mouth," not in the axe. That is where the rest of the poem will keep locating the Mighty-Spirit God's weakness.

When the magic was loosed — clouds spat, mist dimmed;

Parallel halves split at the semicolon: 噴雲曖霧 here, 播土揚沙 next line. This is the standard weather of a magic duel — cloud, mist, dust, sand.

when the hands were opened — earth flung, sand scattered.

Four verbs in six characters, each with its object: fling earth, scatter sand. The line has no subject at all, so the opened hands of the clause before do all of it.

The heavenly general's powers had their method indeed,

天將 takes 將 as jiàng. The couplet weighs one thing against the other: 就有道, the general has method, versus 實無涯, the monkey has no limit.

but the Monkey King's transformations were truly without bound.

化 is huà in 變化, transformation. 無涯 is the sea-word for boundlessness, and it answers a line that had measured the opponent by his method. A technique can be described. This cannot.

The cudgel raised was like a dragon at play in water,

The matched pictures: the cudgel is a 龍戲水, a dragon at play in water; the axe a 鳳穿花, a phoenix threading flowers. 似 is sì.

the axe coming on like a phoenix threading flowers.

鳳穿花 gives the axe a bird's path through blossom — sideways, cutting, decorative. Against the dragon of the matching line it is the lighter and quicker of the two figures, and the less dangerous.

Mighty-Spirit's fame was renowned across the world,

傳天下, "renowned across the world." The couplet raises that fame only to knock it down in the next line with 不如他, "no match for him." 不如 takes bù before the rising tone of 如.

but his skill, it turned out, was no match for the other's:

原來 marks a discovery the narrator has been sitting on. This is the poem's one flat verdict, and it arrives in the plainest words in the whole passage.

the Great Sage lightly swung his iron cudgel,

輕輕掄鐵棒, "lightly swung the iron cudgel." The ease is the entire point.

and one touch to the head left him numb from head to foot.

In 著頭一下 the 著 is zhuó, "onto, landing on" — the same reading it takes when Wukong aims a blow at Nezha's arm later in the chapter — and 一下 takes yí before the falling tone of 下. The poem ends on one almost casual tap that settles everything: 滿身麻, numb from head to foot.