Albert Barnes Commentary Psalms 18:38

"I will smite them through, so that they shall not be able to rise: They shall fall under my feet." — Psalms 18:38 (ASV)

I have wounded them...—I have so weakened them—so entirely prostrated them—that they were not able to rally again. This does not refer so much to wounds inflicted on individuals in the hostile ranks as to the entire host or army. It was so weakened that it could not again be put in battle array. The idea is that of successful pursuit and conquest.

They are fallen under my feet—I have completely trodden them down—a common mode of denoting entire victory, (Psalms 119:118; Isaiah 25:10; Lamentations 1:15; Daniel 8:13; Luke 21:24).