So what exactly are you saying is happening? Brennus is dragging his spear back and the head cuts your character's leg, as he retreats. Meanwhile he raises his shield for cover and her spear slams into the shield, by which point he has finished stepping backward and shoots his spear out again.

If I get this right your characters shield is in front of her leg or something at this point, causing my character's spear to get caught in some fashion as he drags it back? She is then moving her spear after it slammed into his shield and striking at his arm, which is outstretched because her shield caught his spear?

Seems unlikely without your character purposefully trying to capture his spear, which would be difficult.