This is another prompt wherein pre-writing will serve you well. The scene that you’re going to write is just one piece of a larger narrative that you won’t have to write for the purposes of this prompt (although we certainly won’t stop you from doing so!). Start by outlining your character and the monster he or she will face. Give your character some history and personality traits. Then think about the monster and your character’s relation to it. Is this a good knight slaying a dragon? Is it a young mother battling a pill addiction? Take this wherever it wants to go.
You might also think about the moment after. What will happen when your scene ends? While it might be tempting to place this scene at the very end of the story, try to leave room for more. One day, perhaps you might even come back to this idea!