Here "head" is a metonym for the most important part. It is implied that Rezin is only a man, and therefore cannot stop Yahweh's plan. This can be stated explicitly. Alternate translation: "the king of Damascus is Rezin, who is only a man" (See: [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/figs-explicit]])
Here "Ephraim" refers to all of the northern kingdom of Israel. This can be stated in active form. Alternate translation: "an army will destroy Ephraim, and there will no longer be a people of Israel" (See: [[rc://en/ta/man/jit/figs-activepassive]])