In order to fight the Boss, you have to eliminate all of the hitmen. This will make the Boss battle much easier. When you have killed all of the hitmen, enter the confession room on the left. This is the one that you pray in during the first level;s loading screen. The priest will be in the confession room directly next to you, being held captive by the Russian general. The only way to get the general out is to crouch down and shoot a bullet at the heart on the confession booth's inside wall. A short intermission will start and the general will come out, firing an SP12. Use any automatic rifle and fire, making sure every bullet connects. Make sure you have the full amount of ammunition equipped in the gun. He should die within a few seconds.