What Bible verse is that? Mine says HE shall crush the snakes head.
It was precisely she (ipsa) who "with the grace of almighty God" (Lumen Gentium, 56) and with her "immaculate foot" (Pius IX, Ineffabilis Deus) crushed the head of the infernal serpent.
Now Mary has an immaculate foot in which SHE crushed the head of the infernal serpent. She is sounding more and more like a deity to me.
Second, is "with the grace of Almighty God" a synonym for "single-handed" in your book?
Third, you might have missed the first axiom of Catholic Marian studies, that every revealed OT prophecy or fact "finds its realization in Jesus." That means that everything said of Mary is predicated on Christ's power, which is manifest in His Incarnation, in which Mary His Mother is the unique co-operator.
Did you get that? It's the most important fact you can know about Marian studies.
You might want to go back and read everything from the beginning again, with that hermeneutical key firmly in mind.
What it means is that whether the ipsa in Genesis 3:15 refers to "the Woman" as its precedent, or to "her seed" (which could be male or female or neuter, singular or collectively plural) it is Christ who is the victor over sin and death, and thus Christ who definitively crushes Satan's head.
In other words, even if "ipsa" is grammatically applied to refer to the Woman, she is cooperating in this act by virtue of her maternity, that is, by providing for the incarnation of God's son, Jesus Christ. It is her Seed, her Son, God's Son, Who by His Passion and Death, and His glorious Resurrection, accomplishes the redemption of the world.
BTW, the Catholic Bible on my desk right now, the New American Bible, translates "ipsa" as "he" (rather than the grammatically-plausible "he", "it" or "they").
The King James Bible (KJV), as you can see here (LINK), translates "ipsa" as "it". This is also true of the Authorized King James Version (AKJV) and the Orthodox Jewish Bible (as you can see AKJV and OJB versions (LINK)
The oldest of the translations, the Wycliffe version (LINK) translates it two ways: as "her" and as "her seed."
Most of the rest say "he" and "his."
All of this goes to show two things. It shows that the word "ipsa" cannot, from the text itself, be translated dispositively as regards its precedent-word. And it shows that whether it refers to he, she, it, they, or her seed, it points to Christ (the Woman's offspring) carrying out this saving act.
This is made possible by His Incarnation, of which Mary was the key cooperator. Hence His victory, as Son of Man, Son of God, is also a victory for the Woman in which she is uniquely associated by His grace as Mother of the Incarnate Word.