I've been having some trouble with that gorillaboss in Chapter 4, right at the room where you have to use that mounted gun to kill it. I'm already regretting starting this on hard(it was like I hit a brick wall in terms of difficulty at this point in the game.)

I know I'm supposed to shoot it in the head, but it's nearly impossible to get it to stand still for 3 seconds after stunning it. Any pointers would be nice.

Load up as much ammo into the turret as you can when you enter the room. If you're really fast you can get 3 cases into it before he tries to actually attack you at the turret. 
 
For the most part he should be pre-occupied with your teammate.  Honestly, if you're shooting him with the turret you don't have to be precise and hit him in the head. He will take damage as long as you hit him cleanly somewhere on his body. 
 
Also you can request that your teammate add more ammo into the turret so you don't have to, 
 
So best bet is to keep hitting him with the turret and when he approaches you at the turret or you run out of ammo, Lure him out in the open away from the turret and tell your teammate and refill the ammo. Rinse and repeat.
 
You can also shock blast him to stun him for a few seconds as well which can come in handy.

@hbkdx12

Thanks. I will try that out right now.

Edit: Hahaha! I did it! Thanks again, man.

I wasn't aware that you could load multiple cases of ammo into the machine. That tip really helped. Now I can finally get on with my game.

There is no way to switch the difficulty?

I had to use decoy grenades to distract him as the team mate most of the time does a terrible job of doing so. Took me forever to realize this, easily the most frustrating boss in the game and they all really bugged me.