The vanguard brought his men up together.

Six characters to turn a duel into a battle. 帥 is used as a verb here, to lead troops, and the novel keeps up its military vocabulary for a pack of animals coming out of a hole.

This one bout, a confused mêlée on level ground in the mountains, was truly a fine slaughter:

一場 counts a bout or an episode, and here 場 is cháng — the event-measure reading, against the place reading the same character took for the mountain clearing a few passages back. 真是好殺 is the standard cue that a battle description in verse is about to begin: truly a fine killing.

A monk from the great land of the eastern earth, from the upper kingdom; going west to the ultimate bliss to fetch the true scriptures.

The rhapsody opens by naming the stakes rather than the fighters. 極樂 is the Land of Ultimate Bliss, the Pure Land of the west — and the ordinary word for happiness sits inside it.

A great leopard of the Southern Mountain breathing wind and fog; blocking the road in the deep hills, alone showing what it can do.

And there it is, stated outright in the second line: the fiend is a leopard. The battle description tells the reader what the fighters do not know yet, which is how these set-pieces usually work — the verse is the narrator's, not anybody's in the scene.

He laid clever plans and played the sharp one, and in his ignorance took the great Tang Monk by mistake.

乖伶 is quick-witted and sly. The judgement in this line is not that the fiend was wicked but that he was ignorant — he did not know what he was taking, which is the mistake the whole chapter turns on.