Many bosses in A Link to the Past have eyes as weak points. This boss is one of them. Vitreous has several small eyeballs protecting it, and it will send the small eyeballs to attack Link. The most efficient way to kill these eyeballs is to attack them with your sword or shoot arrows at them. A neat strategy for this phase is the go to the bottom-right corner of the room and face forward. Since Link swings his sword from right to left, keep button-mashing the sword button and all the small eyes will be defeated easily. Once all but four eyeballs are defeated, Vitreous will come out of its swamp and bounce around the room trying to attack Link. Run around in circles to avoid it while attacking it with your sword or shooting arrows at it. After several hits, Vitreous will be vanquished, and the sixth Crystal is within Link's grasp.