I beat the first boss on max difficulty using a pacifist character, up to that point all my upgrades went into the 10MM pistol and since I had just about never used it I had oodles of ammo. There's plenty of weapons in the arena too. throw some grenades or use the PED to disable him then start unloading headshots into his head. You can save during boss fights so I set up a string of 3, if I was about to die I would jump to the last save. Probably took me 6-10 tries for my character that had no damage or resistance upgrades. Shoot my stealth character didn't even have the cloak or use silent foot steps. I beat that game with 10 extra praxis points.