I will put enmity between you and the woman, between her seed and yours.
-Mary was at enmity for as long as she was the woman.
-The enmity is attributed to the offspring – Christ – as much as to the woman. But it is beyond question that Christ’s enmity extends to the whole of his life without exception.
-The Fathers and writers…saying, “I will put enmities between you and the woman, between your seed and her seed” — taught that by this divine prophecy the merciful Redeemer of mankind, Jesus Christ, the only begotten Son of God, was clearly foretold: That his most Blessed Mother, the Virgin Mary, was prophetically indicated; and, at the same time, the very enmity of both against the evil one was significantly expressed. Hence, just as Christ, the Mediator between God and man, assumed human nature, blotted the handwriting of the decree that stood against us, and fastened it triumphantly to the cross, so the most holy Virgin, united with him by a most intimate and indissoluble bond, was, with him and through him, eternally at enmity with the evil serpent.
Pius IX, Ineffabilis Deus
Just Thomism 