I beat him at 24. Besides using a stim, maybe an adrenal, and spamming the best attacks as soon as their cooldown would allow, there was one thing I did that made a big difference in beating him. Before you approach him to fight, walk AROUND him, so that should you get tossed around by him, you won't have anywhere to go but maybe hit a wall. That way you can stay close throughout the fight and keep the attack on him.