It's important we understand the source of all grace and power comes from God. And God alone. Yet God can and does choose others to help to act for His will.
So understanding this, we need to be clear it's by the power of God
that the devil was defeated.
But we can also see the Blessed Virgin Mary's body became the living vessel that Jesus Christ came into the world. Thus Mary has a very special bond with her son Jesus Christ.
As it is alluded to by Pope Pius IX, it can be interpreted that God symbolically chose The Blessed Virgin Mary's foot to be part of the representation of the defeat of the devil.
By Mary's obedience to God, and by the grace of God the Blessed Virgin Mary became an instrument of the Lord.
As the Virgin Mary faithfully and clearly stated,
"I am the handmaid of the Lord, let it be done unto me according to your word".
Pope Pius IX on December 8, 1854.
Interpreters of the Sacred Scripture God announced his merciful remedies
prepared for the regeneration of mankind -- words by which he crushed the audacity of the deceitful serpent
and wondrously raised up the hope of our race, 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, and most completely triumphed over him, and thus crushed his head with her immaculate foot.