My strategy: Have Youmu as commander, and open by having her lower RES while Byakuren controls Kaguya. As soon as you get a with Kaguya under your control, run from the battle. You'll fail to escape, but Kaguya actually will run away, leaving just Eirin. Eirin will then start spamming Hourai Elixer, which will fully heal her, so have Sakuya set up her heal-blocking field skill. You'll probably want Alice to Marionette Parrar to reduce damage from Eirin's occasional non-elixer moves, and Reimu to perform healing before and between Sakuya's field move. Have another attacker in the fifth slot, and have Byakuren buff up her and Sakuya (buff ATK and ACC), then just have them attack until Eirin dies.
The downside to this strategy is that you won't get EXP from Kaguya, nor will you get her bestiary entry, so if you really care about that, you'll need a different strategy, but if you're willing to miss that, the battle becomes a lot easier.
