Try having an item called Energy Bank, an item that allows you to access to mass amounts of magic energy for a brief period of time. When the fight starts, use the Energy Bank, then switch to the Soul Reaver. Swipe away at the Boss as quickly as you can. 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 won't 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.
Activate the following cheats by entering the corresponding codes:
CHEAT FUNCTION - CHEAT CODE Full health - ATTACK, SHIFT(2), FORWARD, ATTACK, CENTER, ENTER Full Magic - RIGHT(2), ATTACK, ACTION, UP, DOWN, RIGHT, LEFT View Dark Diary - LEFT, RIGHT, ATTACK, ACTION, UP, DOWN, RIGHT, LEFT