Grab the sword, but do not attempt to hit it when you get the first chance. When the basilisk gets very tall and green appears from its mouth, use the sword on the black diamond on his chest while he is up. To do that, make sure you lock on then attack. Continue to do this and he will be defeated soon. Note: Only attack him while his head is far above the ground. You can use either Flipendo or Incendio (recommended). Let get fully powered, aim at its scales (not its head) and release the button. After five to ten hits, you should defeat it. Also, do not touch the snake or let it touch you. If you do, you will be poisoned. Watch out -- it will get mad and "stomp" to make the stalactites fall. After losing a third of its health, it will start to shoot poison out of its mouth.