Hey.Just dropping by to tell you a few things that probably have been said:
-Change the shotsheet.Because,really,fangame with default bullets?Sounds like Alternative Age to me.
-Boss needs to stay on top of screen , that way the player won't get walled.
I watched your video,and a few things about Cirno:
-Make her nonspells and spells canon,or based off the canon ones.You'll see,it's fun to try to replicate but add your touch.
-In her second nonspell,where she fire the shot that creates other shots,how about you make the shots have a random speed (their speed+rand(-0.5,0.5)),that way it'll be easier for the player to stay un the global field around Cirno instead of having to do ridiculous detours to hit her.
-More importantly,make her move!And give her pauses in her nonspell.
That's pretty much it.
Also,don't go option C.Sukuna did it,and...,it sucked.
Hope this helps.