"The powers of Hell fear the name and protection of Mary."

Men do not fear a powerful hostile army as much as the powers of hell fear the name and protection of Mary.

-St. Bonaventure

“God has established not just one enmity but ‘enmities’, and not only between Mary and Satan but between her race and his race. That is, God has put enmities, antipathies and hatreds between the true children and servants of the Blessed Virgin and the children and slaves of the devil. They have no love and no sympathy for each other. The children of Belial, the slaves of Satan, the friends of the world, - for they are all one and the same - have always persecuted and will persecute more than ever in the future those who belong to the Blessed Virgin, just as Cain of old persecuted his brother Abel, and Esau his brother Jacob. These are the types of the wicked and of the just. But the humble Mary will always triumph over Satan, the proud one, and so great will be her victory that she will crush his head, the very seat of his pride. She will unmask his serpent's cunning and expose his wicked plots. She will scatter to the winds his devilish plans and to the end of time will keep her faithful servants safe from his cruel claws.

But Mary's power over the evil spirits will especially shine forth in the latter times, when Satan will lie in wait for her heel, that is, for her humble servants and her poor children whom she will rouse to fight against him. In the eyes of the world they will be little and poor and, like the heel, lowly in the eyes of all, down-trodden and crushed as is the heel by the other parts of the body. But in compensation for this they will be rich in God's graces, which will be abundantly bestowed on them by Mary. They will be great and exalted before God in holiness. They will be superior to all creatures by their great zeal and so strongly will they be supported by divine assistance that, in union with Mary, they will crush the head of Satan with their heel, that is, their humility, and bring victory to Jesus Christ.” (Treatise On True Devotion To The Blessed Virgin, 54).

I have always viewed the sling which David used to slay Goliath as a harbinger, a sort of figure or sign pointing to the Holy Rosary which would be given to the faithful by Our Lady and by which they would “crush the head of Satan” – “the very seat of his pride” as Montfort puts it. We read in 1 Samuel 17:

“With his shield-bearer marching before him, the Philistine also advanced closer and closer to David. When he had sized David up, and seen that he was youthful and ruddy, and handsome in appearance, he held him in contempt. The Philistine said to David, ‘Am I a dog that you come against me with a staff?’ Then the Philistine cursed David by his gods and said to him, ‘Come here to me, and I will leave your flesh for the birds of the air and the beasts of the field.’ David answered him: ‘You come against me with sword and spear and scimitar, but I come against you in the name of the Lord of hosts, the God of the armies of Israel that you have insulted. Today the Lord shall deliver you into my hand; I will strike you down and cut off your head. This very day I will leave your corpse and the corpses of the Philistine army for the birds of the air and the beasts of the field; thus the whole land shall learn that Israel has a God. All this multitude, too, shall learn that it is not by sword or spear that the Lord saves. For the battle is the Lord’s, and he shall deliver you into our hands’…David put his hand into the bag and took out a stone, hurled it with the sling, and struck the Philistine on the forehead. The stone embedded itself in his brow, and he fell prostrate on the ground. Thus David overcame the Philistine with sling and stone..” (1 Samuel 17: 41-47, 49-50).

The Devil is the adversary of faithful Christians. And, like Goliath, he seems to cast a big shadow. But when we approach him on the battlefield [“For our struggle is not with flesh and blood but with the principalities, with the powers, with the world rulers of this present darkness, with the evil spirits in the heavens,” Eph 6:12] we have our own sling as a weapon – the Holy Rosary. And every bead, every Hail Mary prayerfully recited, is hurled as a weapon against the Devil’s head, the seat of his pride. Therefore, Montfort exhorts us, “So arm yourself with the arms of God, with the holy Rosary, and you will crush the devil’s head and stand firm in the face of all his temptations. That is why even a pair of rosary beads is so terrible to the devil, and why the saints have used them to fetter him and drive him from the bodies of those who were possessed.” (The Secret of the Holy Rosary, 85).

Related reading here.