I was the last person alive on my team during the boss fight. They'd gotten him down maybe 75% of his life before they all died.
I spent the next 10 to 15 minutes kiting him around the second floor of a building overlooking a courtyard. He does so much damage on this difficulty that to let him shoot you more than 5 times is to die, so all you can do is fire off a couple shots at him as he rounds the corner, then run. You can't stand still for very long because he'll either grenade you back to the stone age, cover you in clouds of nerve gas, or rush at you and carve you up like a roast with his claws. So I run, I shoot, I run, I shoot, I get clipped, I heal, I run, I reload, I shoot, I get clipped, I run, I heal....
Constantly I ran out of ammo and I'd have to leave the kiting spot and collect more, which gets really dicey since there's big open spaces to run through, the perfect place for him to light you up with his guns. I'd run out, grab my ammo, then haul ass back to the kite spot, usually with him right behind me. Being medic with a run speed boost and potent self-heals, at the cost of zero bonuses to damage, is all that kept me alive. You move slower the lower your health is, so to getting hurt at all for most any other class results in you being too slow to run from him. Being able to get back up to full speed in a matter of moments with healing is what kept me going.
A couple times while kiting he'd suddenly change direction on me (or I'd lap him more likely), and I'd have to jump out a second story window and run back to the kite spot again, usually with him chasing or chucking grenades ahead of me, which I'd have to sprint through as they started to explode.
After what felt like a half hour, but was probably closer to 10 minutes, I finally killed him with 1 last bullet. I felt like I had just run a marathon (and in terms of meters traveled, my character probably did!)