GoS Approach

(I forgot where Aya went, oops)

EF1
===

Party: Remilia, Sakuya, Nitori, Mokou, Cirno

- Remilia in the front to draw all hits; her high EVA lets her avoid all the physical attacks
- Even when Remilia gets hit, she can recover ridiculous HP with her draining skill
- Nitori can use Hyper Camouflage whenever it's ready to help others survive
- Sakuya uses Suction World after every Resentment Zone, both to nullify it and to allow some degree of healing
- Mokou and Sakuya dish out great damage
- Cirno for Freeze Chaser fuel; Use Minus K and Ice Beam for bottom head; Any freezing skill for top head because FREEZE chance is nice, despite 0 damage only
- I haven't used Yuugi, but I apparently suggested her for EF1 since she's useless for EF2 beyond sponging hits

EF2
===

Party: Marisa, Sanae, Youmu, Reisen, Yuugi

- After Sanae's Wind of Prayers, Marisa can OHKO all mirrors (and most summons) with Triple Spark
- In addition to buffing, healing, and reviving, Sanae can also inflict Toxic on the boss, which is extremely potent
- Youmu and Reisen can dish out really heavy damage when there are no mirrors
- Reisen is especially awesome here, being able to inflict her own debuffs and abuse her slayers
- Some summons also use buffs on the boss, which Reisen can dispel while abusing them for her own slayer
- Yuugi is pretty much useless but she can sponge hits at least

EF3
===

Party: Lithos, Byakuren, Satori, Reimu, Alice, Commander Patchouli

- Reimu's only purpose is to set up barriers whenever they're ready, and heal people whenever they're not
- Byakuren is in charge of buffs and can act as a secondary healer after Reimu
- Satori should spend her bombs on Starlight Barrier (and maybe stuff like Reflect Wall); don't waste valuable Bomb CD on stuff like Punish the Judgment
- When not bombing, Satori should hit hard with a powerful Medoroa or emergency heal if Reimu and Byakuren are busy
- Satori's Medoroa should be optimized as much as possible; This includes equipping Es, Insight IV+V, Wisdom IV, maxing relevant trees, and utilizing Byakuren's MATK++ and hopefully ALL++ too
- Patchy should activate Royal Diamond Ring whenever Lithos fires Cosmic Supercluster while Satori fires boosted Medoroa; Lithos should not fire Supercluster outside of this situation
- Alice's role is self-explanatory; Abuse Little Legion well
- Byakuren and Reimu can get opportunities to attack too, since you're not always in a MUST-HEAL situation
- I used Power Library for this, with Lithos and Alice in the front; Hyper Library may produce better results