Defeating the Dark Entity
Make sure you have an item called Energy Bank. This item allows you to access to mass amounts of magic energy for a brief period of time. When the fight begins, use the Energy Bank, then switch to the Soul Reaver. Swipe away at the Boss as quickly as possible. He will go down in two hits before he has a chance to do anything. Note: Make sure you do not have a two-handed weapon like the Soul Reaver equipped or you will not be able to use the Energy Bank (or any other items). Alternately, slash the Dark Entity repeatedly with the flame sword when it is on screen and casting the magical barrier once in awhile.