This is a DPS race; augment your damage to full. The Dark Knight has a major Achilles heel that isn't immediately apparent and will be a huge help. His attacks don't hit directly in front of him.
Stay close -- like stupidly close -- and get off fast knife strikes. When he leaps away, dodge roll close to avoid his long-range attack and then stay in tight quarters to constantly attack.
Your goal is to kill him before the timer hits 0. However, if you let the timer run down and darkness overtakes the screen, you can use the Light Of The Phoenix ability to get a small area of visibility.