[X] Snipe him from a distance with bows: It has the advantage of giving them a surprise strike, but carries the risk of the arrows being sent off course by the wind of the vortex, or blown out of the air by the lightning.
[X] Divide and Conquer: Have two forces strike at him, a frontal assault and then a pincer attack while he's distracted. With his focus split three ways you may manage to take him out.