Simply bait him around the area shooting with a pistol when safe to; either when you have made a bit of space or while he is attacking. Try not allow him to get too close as he may grab you for a instakill attack, this is why you need a few knifes to counter against (try not to waste grenades though).
Don't waste any other ammo, pistol is pretty weak against basic enemies, but they stand up well against this boss. This battle is all about keeping your distance!
![](https://i.ytimg.com/vi/CNsJHvg1_fM/maxresdefault.jpg)