先下手為強 Strike First And Be Strong
行者暗算道：「不趁此時下手他，還到幾時？正是：『先下手為強，後下手遭殃。』」就把手一叉，腰一躬，一跳跳起來，現出原身法像，掄起金箍鐵棒，劈頭就打。行者暗算道：「不趁此时下手他，还到几时？正是：『先下手为强，后下手遭殃。』」就把手一叉，腰一躬，一跳跳起来，现出原身法像，抡起金箍铁棒，劈头就打。
逐句解 Line by line
Pilgrim reckoned to himself, "If I do not take him now, when am I to do it?
還 is hái. 暗算 is to calculate silently — and it also means to plot against someone, which is what he is doing.
It is just as they say: 'He who strikes first is the strong one; he who strikes second meets disaster.'"
為 is wéi, the copula — is the strong one — not the wèi of on account of. The proverb is still in daily use in Chinese and means exactly what it says.
So he crossed his hands, arched his back, sprang up with a leap, showed forth his original body and its magic form, swung up the gold-banded iron cudgel and struck straight at her head.
Five actions in a single breath, each of three or four characters, so the sentence accelerates as it goes. 原身法像 is his true shape — the monkey, dropped back into after the boy.
The disguise comes off in the middle of a sentence, and the prose speeds up to match: crossed hands, arched back, a leap, the true body, the cudgel, the blow.
The proverb he quotes to justify it is entirely amoral and he quotes it approvingly. Strike first. This novel's hero settles a question of timing with a piece of street wisdom about getting your blow in before the other party does.
It is worth noticing that he waited. He let her walk him out of the hall and take him to the ground rather than strike in front of the Buddha, and only now, with the method proved and nobody watching, does he move.