搗蒜打 The Garlic Pounding Blow
若老孫使鐵棒往下就打，這叫做『搗蒜打』，打便打死了，只是壞了老孫的名頭。」那行者一生豪傑，再不曉得暗算計人。若老孙使铁棒往下就打，这叫做『捣蒜打』，打便打死了，只是坏了老孙的名头。」那行者一生豪杰，再不晓得暗算计人。
逐句解 Line by line
If I were to take my iron rod and simply strike downward, that is what they call the garlic-pounding blow. It would kill him well enough, but it would spoil old Sun's good name."
搗蒜打 — the garlic-pounding blow — is a straight downward smash, named after the motion of a pestle going into a mortar, and the novel puts it in quotation marks as a bit of slang. The objection is not that it would fail. It is that it would work, from behind, on a creature who has not seen him.
That Pilgrim has been a hero all his life and never has learned how to do a man in by stealth.
The narrator steps in to vouch for him, which is unusual and worth marking. 暗算計人 is to plot against someone in the dark. 曉 takes 得 at dé, its full reading, after a verb of knowing.
This is the novel telling you, in its own voice, that Pilgrim does not do things behind people's backs. Hold on to it. Within a page he will send his brother into an ambush he knows about and has not mentioned.
The two statements are not quite a contradiction, and the gap between them is where the chapter lives. He will not kill a demon from behind. He will absolutely trick a colleague from behind, and the novel does not think of the second as the same kind of act.