UnMetal

This occurs about Midway through Chapter 9. There will be a toilet prior to this boss so make sure you use it in-case you need to reload.

It might also be worth mentioning that I took the skill for guns to do 30% more damage (I believe you get the option for this at level 6?) though I don't believe it's mandatory, though it may help.

Upon entering the arena you're going to want to unload as many pistol shots straight into the boss as you can. Once he teleports towards you and slashes, you're going to want to roll out the way to avoid the consecutive strike and again unload your gun into him. Roll twice if you need to get a reload in. You might also need to heal depending on your difficulty and the bleed damage.

Ocassionally he throws fans that spread across the arena but it's possible to dodge these and keep unloading on him.

Just play it super aggressive and keep unloading on him. The quicker you kill him, the less chance he has to heal. You'll notice if he heals because he drinks from a flask and has a green healing aura around him, similar to when you use a med-kit.