Normaly her sheilds would have deflected such a hit from a plasma weapon. But due to the previous fight, and her shoulder injury, she was already set up for failure. It was more than amazing that any human could withstand such injuries. The plasma fire was enough to knock any grown man off his feet, but she just took the hit like nothing. She drifted in and out of conciousness. She was angered, not at the covenant but at herself. She shouldn't have let herself get this bad.