Premature Finale Achievement GuideThis boss fight is story related and can not be missed. When fighting Stefano Valentini it's best to keep moving and keep an eye on when he's about to take a picture so you can quickly get behind a wall. Good weapons to use are the Sniper Rifle, Sawed-Off Shotgun and the Semi-Automatic Handgun. In the second area you'll want to avoid the big yellow floating orbs since they explode if you get near them.
Just keep running and shooting and get out of sight when a photo is about to be taken and this will be no problem.
Hope this helped.