The writer compares soldiers to "fattened bulls" because the soldiers are well cared for by the Egyptians just as a farmer takes care of a bull and makes it fat. (See: [[rc://en/ta/man/jit/figs-simile]])
To "stand together" is an idiom meaning to remain united. The writer is saying that the soldiers will not fight as a unit but will run away thinking only of saving themselves. (See: [[rc://en/ta/man/jit/figs-idiom]])
This speaks of a day as if it travels and arrives in a location. Alternate translation: "they will experience disaster on that day" (See: [[rc://en/ta/man/jit/figs-metaphor]])