In Dark Matters first form when he starts charging his sword get extremely close to him and swing your sword at the moment he releases the ball of energy. It will do triple the damage to him.

In Dark Matters second form he will first attack you with four oval shaped balls of energy which will explode. Before he flings them at you get behind him and swing your sword at the balls directly behind him (try to hit them both). If successful 1/3 of his energy should be gone and you'll have him defeated in no time.