The word "you" refers to Nineveh. Nahum speaks of an army or military leader destroying Nineveh as if he were to shatter Nineveh like one would shatter a clay pot. See how you translated this in [Nahum 2:1](./01.md). Alternate translation: "The one who will destroy you" (See: [[rc://en/ta/man/translate/figs-metaphor]])
This can be stated in active form. Alternate translation: "The attackers make ready the large shield to protect themselves" (See: [[rc://en/ta/man/translate/figs-activepassive]])
This refers to a large cover that those who besieged a city would set up over themselves and their battering rams to protect themselves against the arrows and other projectiles with which the people in the city would attack them.