this stage's boss is not hard at all. the hard part is the long dungeon (beware hydra - it is stronger than boss)
this boss has very high defend and weak to water.
Recommended team: Sanae, Marisa, Patchouli, Reimu ,Alice and Mokou as commander.
-Patchouli:run 25points on Spirit Magic and magic mastery. the rest of point(if you have) run in int. equip water skill.
-Marisa:run 25points of Magic Mushrooms and 20 points on Astronomy. the rest of point(if you have) run in int. equip water skill include water field and active Elemental.
-Reimu:run 25points on Barriers. equip healing skill.
-Sanae:run 25points on Wind Priestess. equip buff skills(MATK up,EVA up,max hp up).
-Alice:run 25points on Manipulation and Craft. equip with Little Legion and Marionette Pareil.
fill up water gauge by active Elemental first turn and begin with MATK up buff. keep buffing when need and use reimu to heal the whole party (with 25points on Barriers so she can heal all party). when boss change the field ,you can use Arctic Chill to change the field back to water field so it nagate boss's field effect and increase water damage. keep attacking with water skill.
on last turn(before the boss die),it will cast last word that deal heavy damage to party so before kill it, using Alice's Little Legion or Mokou 3bombs commander skill