Silent Hill Homecoming Boss: Scarlet
Use the crowbar and hit Scarlet's legs.
Cracks will appear all the way up her leg when it is damaged completely. Do this to both legs.
Once the legs are damaged enough, she will fall to the ground. Attack her arms and when you've done enough damage to them you will see a context clue appear. After a bit she will stand up and you'll have to start at step one again, except the legs will already be mostly damaged.
Once you break all the limbs and the context clue appears, press the buttons indicated to defeat the first form.
Now Scarlet will change into a spider-like creature. Dodge it's attacks and hit it with the crowbar.
It will climb the wall and then pounce at you out of the dark. If you can dodge this, then you have a good shot of using your crowbar on her (more context clues.)
Once you do this three times you will defeat Scarlet.
You will see a key sink into the blood.
Then you will begin to sink into the blood as well.