Annie injects herself with the poison containing uranium (the only substance that makes the poison work against the monster) that was prepared by Nodin in order to kill the monster. The monster kills Annie and dies itself as soon as it comes in contact with the uranium in the poison.
To kill the monster, Annie self-administers a poison made by Nodin, which contains uranium. This poison is the only thing that can harm the creature.  The monster then kills Annie and dies instantly when it comes into contact with the uranium in the poison.