Oh him? He's not too hard either. Stand away from the center of the arena, keep your arrows out (FPV) and circle.
Look for him coming forward in one of the pictures. Or two of the pictures. It's just a pixelly black spot at first but it gets bigger.Then keep your arrows trained on him until the portal appears, and shoot him when he starts to come out.
Once you kill his horse, Z-target him and switch to your sword and shield, and play tennis. When he throws an energy ball swing your sword and send it back. It's good practice for Ganondorf; he does it too.