The start of the mission returns you to the prologue, just before the horde starts their attack. Wait for the members of the horde to approach, and engage only the ones that attack you directly. When the demon attacks, knocking the car over, Miller will rush to your aid and repel the demon. The horde will disperse slightly, but stragglers will still attack.
Closely follow Miller to the nearby shed, which will provide some cover from the remaining enemies. You should reach the tower without difficulty, and take the elevator to the top.
The elevator ride was a bit rough, causing you to be thrown out to the ledge.
Head to the right, and climb over the obstacle. Run around the side of the tower, until you reach a staircase to the upper floor. When you reach Miller, a demon will break through the side and attack. Shoot the demon once with your current weapon (including the knife) to kill it.
You now need to climb to the top by yourself. Continue along the side until you reach a ladder. Climb it, and tapwhen prompted. Look for a doorway and head down the stairs to collect the last two filters in the game. Head up the stairs, and outside. When you return to the building, you will have to jump across a few platforms, before entering another quicktime event sequence.
Climb to the top of the tower, and place the guidance system on the extended platform. The Dark Ones will start attacking directly, bringing you into a mind battle.
You find yourself in the room with four doors. Walk to each door and open them, passing by the center eack time. On the fourth door, you will be pulled by the Dark One, and thrown out onto the outer platform. Walk back to the tower, and the mind battle will continue.
The battle starts with you in a mind maze.
The wall in front will only open when you look away. Do so, and walk down the newly opened corridor. When the Dark One approached from behind, run forward, until you see a flash with another dark one approaching. Turn around and run down the corridor. When you see the lamp within the stone wall, turn around until you encounter the dark one again. Run away, and you will encounter a branch - to the left is a wounded ranger, and to the right is hunter. The left option is a trap as shown by the red glow. Going towards hunter will bring you to the final part of the maze.
The terrain looks like a broken ground. You will have to jump across to the first platform, but can continue running forward. The Dark One will approach from behind. To evade him, take the first branch to the right, and jump across the gaps. The fourth jump requires hugging the right wall, and the final jump will return you to a more normal tunnel.
The Dark One approaches yet again. Take the left turn, and you will eventually stumble. Hunter will give you a pistol to defend yourself. When you turn around, shoot the Dark One until it collapses.
You will return to the tower after this battle. If you obtained sufficient moral points in the prior chapters, you will have the option of disarming the guidance system, or letting the missiles run their course. With either option, the game will be completed.