Defeating Volgin on top of the Shagohod is the same as fighting it the first time. Take out your RPG and aim for the treads. When you hit the treads, you will stun the Shagohod and leave a window open to do damage to Volgin. You cannot hurt him unless you shoot him in the sides of the back. Make sure you are close enough to get behind him and unload with any gun. Do this to make quick work of Volgin.