東方玄夢妖譚 ~ The Nightmare of Rebellion - チルノ 見参!?
All I can say to this fight? Oops! Unrelated note, the boss BGM here is awesome.
Cirno is weak to Lightning and isn't a pushover. If she uses Spirit Focus her attack, she will deliver either Icicle Fall or Perfect Freeze in the next turn BUT if Power Focus was used, she will go for Icicle Fall which will kill your front row party. Perfect Freeze causes damage+Stun on all so take note on that.
Use Sanae, Schwarze Katze and Wandering Female Student to powerup while for elemental defense, use Jason and PuruPuru to lessen the damage on Cirno's ice attacks. Physical wise, she is strong so keep an eye on your party's HP and I cannot stress this enough but: go for Chasers! They'll save your butt!
After the fight, Cirno will join the Doppel gang to avoid getting into more trouble.