“Fight off the barrage of unidentified enemies.”
Oryx has loosen the Taken against you. This includes the taken Knight: Syrok, Word of Crota. First, kill the Taken Psions. They can multiply and do so fairly quickly. If they get a good swarm, then the fight is over from the sheer amount of fire they can bring against you.
Once the Psions are cleared focus ion Syrok. Syrok follows a simple pattern of moving around the far part of the room. He will peroidically loose a barrage of flames from his eye. Strafe away from him as he is doing this. It will help you avoid damage from him. Just keep a good amount of gun fire going at him.