I'm a mom to a 12 year old boy, a wife of 15 years, a teacher for 11 years, and my degree is in psychology/child development.
I would continue to support her and reassure her that you are there to keep her safe. Don't try to use logic and lots of discussion - this isn't about being logical; it's emotional. Maybe something has changed recently in her world that is making her feel less confident and secure. Perhaps, you should try some pretend play where she "attacks" the monster, so that she feels empowered. At any rate, try giving her lots of emotional support when she needs it. Just leave a light on or place a lamp in the bathroom for the next 6 months or a year. For the room darkness, get her a night light, or a dim lamp. I've seen a turtle lamp at Hobby Lobby that gives off a faint yellow glow. I've also seen a lamp that looks like an aquarium with fish scrolling by. When she awakens in the middle of the night, take her back to her room and tuck her into bed.
We ended up having to do the sleeping bag on the floor of our bedroom because our son had night terrors and sleep walking. He didn't decide to start sleeping in his bed until he was almost out of 4th grade. I never thought it would happen, but one day he told me that he wanted to try sleeping in his own bed the next night.
Also, consider any medications that may interfere with sleeping. It turns out that my son's years of night terrors and sleep walking were mostly due to taking Singulair. I never knew that was a possible side effect, and when he turned 12, I said something to the peditrician about him being too old for sleepwalking and he said we should have taken him to a neurologist a long time ago. Long story short, we took him off the Singulair, and he hasn't had a single episode!
Good luck!