God will make Israel to destroy their enemies as the Eastern corn-drag (Isa 28:27, Isa 28:28) bruises out the grain with its teeth, and gives the chaff to the winds to scatter.
teeth - serrated, so as to cut up the straw for fodder and separate the grain from the chaff.
mountains ... hills - kingdoms more or less powerful that were hostile to Israel (Isa 2:14).