The key is to first destroy the crystals without getting damaged significantly. You can shoot at least four crystals from behind the twisted pillar in the back where the boss can't see you. Once the proper battle starts, you know the drill - shoot him in the head, when he's stunned, circle around his back and shoot him in the vulnerable spot on the back. Luckily, if you have quick reflexes, you can keep him stunlocked for several seconds, enough to bring 50% of his health down in one go. When you see him start to turn around and face you, adjust your position so that you always fire perpendicular to him. It's a bit tricky to pull off, but that way you can safely take away a lot of his health, not just score one hit.
Once below 50% health, he gets noticeably faster. Now shooting him in the head is no longer a viable tactic since he snaps out from being stunned before you can circle around his back. The best way is to get really close to him, so that he'll initiate his jumping attack. While he's still in the air, position yourself so that you're far enough and directly behind his back when he lands (just mind his jump backwards after he lands).
Oh, and reduce the shooting mode difficulty to Easy for this battle. That's what helped me. :)
