Spoilers follow for the endgame:
*** Spoiler - click to reveal ***
* Acquire the Syringe by clearing the Infinite Room in the tunnels below the mansion, then progress as normal until near the end of the game.
* In the Bunker, confront Noah and dispatch the four radiation-suit guys.
* Use the Syringe on the monster strapped to the cross.
* Take the Bolt Cutters from the compartment in the cross behind the monster.
* Complete the final boss fight as normal.
* Return from the Bunker to the Sewers and use the door to the Pumping Room to return to the Office.
* On the second floor, go into the Viewing Room and use the VCR with the Experiment Room tape.
* Use the Bolt Cutters on the door to Anna's cell.
* Speak with Anna in the Main Hall.
* Use the Cross-Shaped Key on the Main Hall's exit.