How do you defeat the sorrow in Metal Gear Solid 3

There are two ways to finish that fight. You can't kill him in either (as he's already dead), so don't waste ammo.

Quick way: go prone in the water and drown yourself. When the 'game over' screen appears, open your items menu with L2, and take the revival pill.

Long way: follow the Sorrow all the way to the end, where the 'game over' screen comes up. Take the revival pill. Going all the way to the end gets you the 'ghost' camouflage.