東方冥異伝 / Touhou Meiiden - vs Mystia
Well there isn't much to say about this battle... unless you didn't hit Mystia hard enough during the previous encounters. Otherwise, you're stuck fighting a monster that constantly stunlocks you all the way until it's game over.
I swear, the first time I fought Mystia in this game around 7 years ago, it was a real pain. It took me around 20 to 30 tries before I finally got to down her.
Mystia is:
Strong against Metal, Slashing and Blunt attacks.
Weak against Long-Ranged attacks.