Once you and Hermes combine the two pieces of the stone, the portal will begin glowing and Hermes will yell at you to take up the stone. Grab it and then Sarah will snatch the stone away from you. Hermes will yell at you for a moment and then you have a second or two to shoot Sarah with your gun. When you shoot her, you’ll trigger the final cutscene and unlock the achievement.
(FYI-I used the Alone in the Dark 360 achievement guide, credit goes to Andrew x360a)
http://www.xbox360achievements.org/game/alone-in-the-dark/gu...(I do not own this video. It belongs to a youtube user huzy619)