“Enmity” has the intensity of hostility experienced among nations in warfare (e.g., Ezek 25:15; 35:5) and the level of animosity that results in murder (e.g., Num 35:21). The language of the passage indicates a life-and-death struggle between combatants. “Crush” and “strike” translate the same Hebrew verb šûp (AV, “bruise”) and describe the combatants’ parallel action, but the location of the blow distinguishes the severity and success of the attack. The impact delivered by the offspring of the woman “at the head” is mortal, while the serpent will deliver a blow only “at the heel.” Continuing the imagery of the snake, the strike at the human heel is appropriate for a serpent since it slithers along the ground, while the human foot stomps the head of the vile creature.
Mathews, K. A. Genesis 1-11:26. Vol. 1A. Nashville: Broadman & Holman Publishers, 1996. Print. The New American Commentary.