劈頭就打 The First Blows Are Struck
大聖道：「哥說得是。小弟這一向疏懶，不曾與兄相會，不知這幾年武藝比昔日如何，我兄弟們請演演棍看。」這牛王那容分說，掣混鐵棍，劈頭就打；這大聖持金箍棒，隨手相迎。大圣道：「哥说得是。小弟这一向疏懒，不曾与兄相会，不知这几年武艺比昔日如何，我兄弟们请演演棍看。」这牛王那容分说，掣混铁棍，劈头就打；这大圣持金箍棒，随手相迎。
逐句解 Line by line
The Great Sage said, "My elder brother speaks rightly. Your younger brother has been idle this long while and has not met with his brother; I do not know how the martial art of these last few years compares with what it was. Let us brothers try our staffs and see."
說得 takes the toneless complement marker de. 相會 takes xiāng, the each-other reading. 演演 doubles the verb again, keeping the challenge in the register of a friendly practice bout.
The Bull King would allow no explaining: he whipped out his wrought-iron staff and struck straight at the head; the Great Sage held up his gold-hooped cudgel and met it as it came.
那 is nǎ here, rhetorical: how would he allow it. 相 is xiāng in 相迎 too, not xiàng — the same division the chapter has been making since the Sage's own looks were described. The sentence is built as two matched halves, one for each combatant, with nothing to choose between them.
The Great Sage's acceptance is framed as an invitation to spar. He has been idle, he says; he has not seen his brother in a long time; he wonders how the other's skill is holding up; let them try their staffs and see. Not one word acknowledges that the terms just offered were three bouts or death. He is answering a challenge as though it were a proposal to keep in practice.
The Bull King does not play along. 那容分說 — he would allow no explaining — and the blow comes at the head without preliminary. The two halves of the sentence that follows are cut to identical measure, one combatant each, and the balance is the point: the narrative gives neither of them the advantage of the first stroke, because the poem about to begin will not give either of them the advantage at all.