When you are facing the final boss the Immortal, you must remember to gain two adrenaline points. Use the tactic where when you move from one part of the wall to another, time slows down. Using this tactic you will be able to shoot the Immortal as he darts from one stone column to the other. When you have used this once or twice he will smash the floor and vanish. Next four enemies will come through the windows and you must kill them to gain adrenaline. Once you have done this the Immortal will reapear. Use this tactic again, especially when he has just appeared as it does not take too much effort to aim as he is at a standstill. As earlier doing this once or twice he will take a large ammount of damage and then vanish through the floor. Then another four goons will apppear, kill them to gain adrenaline. Then the Immortal will reappear. Use this tactic once or twice more to serverely damage him.
Now you will enter a cutscene where you will kill the Immortal and find a binary pattern in a rip in your special suit this shows the combination for the Immortal. After this you can bask in the greatness of the credits, an extra trophy which trophy it depends on the difficulty of the game what one and of course you can be another person who's completed the game.