There is only one way to kill a Phantom. Once you have the Phantom Sword, you can hit the in the back and destroy them. A normal sword will not do any damage. You do not get the Phantom Sword until you are ready to fight Bellum. If you still have a normal sword you can still paralyze the Phantoms. While their back is faced towards you, shoot them in the back with an arrow. This can buy you some time to run.