Adam Clarke on Psalms 21:12

Therefore shalt thou make them turn their back - God can in a moment strike the most powerful and numerous army, even in the moment of victory, with panic; and then even the lame, the army which they had nearly routed, shall take the prey, and divide the spoil.

Against the face of them - Thou shalt cause them to turn their backs and fly, as if a volley of arrows had been discharged in their faces. This seems to be the sense of this difficult verse.