here's how you play DRK in a single target boss fight

start in grit, use power slash combo or two, turn grit off.

If there is a ninja/samurai/warrior, wait until they apply slashing debuff, then use DA C&S, otherwise, blow it right away. spam dark arts as much as you can, and keep blood weapon on cooldown. Use delirium whenever possible AND whenever blood weapon is active, otherwise use your blood gauge on bloodspiller. do not use dark passenger. Make sure to always use plunge and salted earth as soon as they are available.

Make sure your MP never gets full. If your MP is full, that means MP wasted that could've been used for more dark arts. Remember, it makes no real difference if you use dark arts in syphon strike or soul eater, so just use dark arts before your MP gets too high. Keep an eye on C&S and make sure you have enough MP before it comes off cooldown to use dark arts on it.