K so here's how you do it. What i figured out is that his armor makes him invincible from all attacks, at first. So what you do is attack his back with a zip kick attack. But you can only do this after he's tried to attack you. He only has two attacks so it's pretty easy to know what he's gonna do.
If he turns red use the left bumper and dash to the side because he's going to charge right at you. Then immediately grapple onto his back and kick him.
If he turns blue that means he's going to jump in the air and do a ground pound. So you have to jump right before he hits the ground. Then you'll have to run around him and grapple his back and kick him again.
Do this enough and you'll see his armor pulse with electricity. So now you can shoot him. Or you can keep grappling if you want to.
Also be careful of the little flying drones as they can be annoying on higher difficulties. To avoid them just keep strafing around the platform.
If you have any other questions on what to do just let me know. I'll be glad to help anybody out :)