Meirin may have only been sixteen at the time, but she had at least ten years of martial arts training under her belt, and she had already begin training her body for fights. That being said, she did get a lot of help from her fellow artist, many who died from their wounds while fighting alongside her. The demons themselves were average strength I guess, but it was mostly the element of surprise that allowed them to do the most damage. If you're wondering why they attacked the monastery in the first place, I'd rather leave that in secret unless you want me to PM you about it. As for the nightmares, they work in part of Meirin's narcolepsy, as they're what keeps her asleep for abnormal times and otherwise messes with her head. If she didn't grow up in the Monastery, she'd probably have gone insane by now. But Meirin has also honed her mind as well as her body, though not enough to completely stop her Nightmares.