Use the following trick to defeat Volgin on Shagohod easily. When the battle begins, run to a safe place where you know Eva will not ride to. Stop, go to your inventory, and equip the RPG or rocket launcher and some sort of "lethal" sniper rifle. Aim at the spinning drill-shaped wheels on the Shagohod. Make sure the rocket hits one of them. You will notice that the Shagohod has stopped. Take out the sniper rifle and aim it at Volgin's head. One shot to the head should take him out.