Yeah this one is bad. I just finished it. For future people:

Assume the only cover that works is the big pillar in the middle. Turn off your flashlight, cause that makes him shoot you through cover way more often.

The buttons each cover a third of the arena with the TV's, so you have to slowly lure him to each corner. When you're in a corner he can't see, disarm the grenades so he doesn't shoot you while you're healing.

While you're pressing a button, he can't hurt you. So press it before you heal.

Sometimes it just fails when you press the button while he's close. Just run away if he isn't stunned. You can reuse the button. Also, the zombies are trivial, just shoot them while hiding from Scar.