徑投東走 He Broke Away Eastward
那老怪戰行者已是難敵，見八戒鈀來，愈覺心慌，敗了陣，將身一幌，化道寒光，徑投東走。他兩個決不放鬆，向東趕來。那老怪战行者已是难敌，见八戒钯来，愈觉心慌，败了阵，将身一幌，化道寒光，径投东走。他两个决不放松，向东赶来。
逐句解 Line by line
The old fiend was already having trouble matching Pilgrim, and when he saw Bajie's rake coming he lost his nerve entirely, broke, gave his body a shake, turned into a streak of cold light and made straight off to the east.
寒光 for the fourth time. 將身一幌, giving the body a single shake, is the novel's fixed gesture for a shape-change — the same motion a wet dog makes, used for a transformation. 徑投東走: he does not flee at random. East is a direction, and it is going to matter within a page.
The two of them would not let up for a moment, and came chasing after him eastward.
Eleven characters to close the scene. The fight is over, nobody has won, and the chapter is now a pursuit heading east — which in this novel's cosmology is the direction of the immortals' isles, and of whoever might be out looking for a missing mount.
The fiend's collapse is caused by arithmetic rather than by any blow. He was losing to one opponent; a second arrives; he leaves. The novel is consistently unromantic about how its fights actually end.
Choosing east is the detail that turns a chase into a plot. A cornered monster running for its life does not usually pick a compass point, and this one is heading somewhere. The next passage explains where, and it is not a refuge.