Cirno and Momiji boss fight:
Have Aya as commander so your party can go first. Have Sanae to buff evade up spell (and other buff spells) and if you like, have Byakuren cast Unquick Momiji. (guarentees at least 90% evade) Have Reimu to cast the barrier spell to protect from getting frozen. (optional) Have Alice to cast Artful enchanter (water element). Then use Alice shield ability to protect your party.
You can either use Marisa, Sakuya, Youmu to attack. Marisa is the recommended choice. You can also slap in Patchouli in the party and add Fire element to the field.
Make sure you take out Cirno first.