CONTAINS SPOILERS
Chapter I: Amy's Boss Fight
Amy will move across the star pentagram in the of the room. Stay in the middle of this star and slightly move so that she doesn't touch you. Don't move in her direction. When she reaches one of the corners, she will make a circle of symbols around her. Don't touch these. Here, you can use your cross while she's still. Later in the fight, she will spawn another gray demon. Dodge by going up/down.
