(Westminster Leningrad Codex)
καὶ ἔχθραν θήσω ἀνὰ μέσον σου καὶ ἀνὰ μέσον τῆς γυναικὸς καὶ ἀνὰ μέσον τοῦ σπέρματός σου καὶ ἀνὰ μέσον τοῦ σπέρματος αὐτῆς· αὐτός σου τηρήσει κεφαλήν, καὶ σὺ τηρήσεις αὐτοῦ πτέρναν (LXX).
From the Pulpit Commentary:
1. Shall crush, trample down - rendering שׁוּפ by torero or conterere (Vulgate, Syriac, Samaritan, Tuch, Baumgarten, Keil, Kalisch).
2. Shall pierce, wound, bite - taking the verb as - שָׁפַפ, to bite (Furst, Calvin).
3. Shall watch, lie in wait = שָׁאַפ (LXX., τηρήσει - Wordsworth suggests as the correct reading τερήσει, from τερέω, perforo, vulnero - Gesenius, Knobel). The word occurs only in two other places in Scripture - Job 9:17; Psalm 139:11 - and in the latter of these the reading is doubtful (cf. Perowne on Psalm in loco). Hence the difficulty of deciding with absolute certainty between these rival interpretations. Psalm 91:13 and Romans 16:20 appear to sanction the first; the second is favored by the application of the same word to the hostile action of the serpent, which is not treading, but biting; the feebleness of the third is its chief objection. Thy head. I.e. the superior part of thee (Calvin), meaning that the serpent would be completely destroyed, the head of the reptile being that part of its body in which a wound was most dangerous, and which the creature itself instinctively protects; or the import of the expression may be, He shall attack thee in a bold and manly way (T. Lewis). And thou shalt bruise his heel. I.e. the inferior part (Calvin), implying that in the conflict he would be wounded, but not destroyed; or 'the biting of the heel may denote the mean, insidious character of the devil's warfare'" (T. Lewis).
"And I will put enmity between thee and the woman, and between the seed of thy son, and the seed of her sons; and it shall be when the sons of the woman keep the commandments of the law, they will be prepared to smite thee upon thy head; but when they forsake the commandments of the law, thou wilt be ready to wound them in their heel. Nevertheless for them there shall be a medicine, but for thee there will be no medicine; and they shall make a remedy for the heel in the days of the King Meshiha" (PsJon Gen 3:15).