In li-taḍribī, the lām is lām al-taʿlīl, marking purpose. The following second-person feminine imperfect is subjunctive and, being one of the five verbal forms, has lost its nūn. Thus the tears are represented as having the purpose of striking the heart.
In bi-sahmayki, sahm means arrow; the dual construct form and the attached -ki yield “with your two arrows.” The arrows evoke the woman’s two eyes or their glances.
This language transfers the experience of emotional pain to the action of an archer. No literal bow, arrow, or physical attack is being alleged; the aim is to show the depth of the inward wound.