Turn their back, i.e. flee away at the first sight of thee, whereby also they will be a fit mark for thine arrows. Or, thou shalt set them as a butt to shoot at, as the like phrase is used, Deuteronomy 7:10 Job 7:20, Job 16:12. Against the face of them; or, against them, the word face being oft redundant.
Related Commentaries of Psalms 21:12
Psalms 21:12
12 Therefore shalt thou make them turn their back, when thou shalt make ready thine arrows upon thy strings against the face of them.