Beat the Magatama of the High God. While my average was ELEVEN levels below the CHALLENGE level for it.
Magatama of the High God is actually easier then its previous incarnation because it doesn't have Black Universe[Reduce one party member's hp to one with very low delay so it can act again almost immediately afterwards].
Special thanks once again go to the Yakumo trio and Byakuren, with Flandre and Tenshi getting honourable mention. With all three Yakumos on the front lines at once, Yukari and Chen can absolutely shred anything without high physical affinity or absurd defenses while Ran piles on the buffs. Mind you, Byakuren gets most of the buffing credit, due to being able to 100% buff the four most important stats, but Ran was still quite useful. Flandre did some great damage, but was too glass cannony to survive more then two Starbow Breaks. As for Tenshi, she just wouldn't die, even at literally one health.
I basically just whaled on the Magatama with Shikigami Ran + Chen alternating with Flandre for the first stage. After it reduced the entire party's hit points to 1 with World Devouring Devastation, things got pretty desperate. Reimu healed my most important members[The Yakumos and Byakuren], but bit it soon after. Flandre, Kasen and Kaguya soon followed. Still, I persevered and the Yakumo beatdown engine was still going strong.
Then it reduced everyone's health to 1 again. When I had no aoe healers. Eirin helped some, but honestly this part was just blind luck. The Magatama used Half Moon Slash[Halve the party's hp] five times in a row. Since Eirin had an ability that healed everyone for 4% health every turn, this was miraculously not lethal. Meanwhile, Yukari and Chen desperately pummeled it while Byakuren tried to debuff its defence. Eirin got dunked along the way, but not before overhealing Yukari into near invincibility. Well, "Near" invincibility. The Magatama finally wised up and killed everyone besides Tenshi, Byakuren and Yukari fairly easily. Yukari didn't last long either, even with her great regen ability. So I had just Byakuren and Tenshi left, and the latter had very low health. Miraculously, the Magatama continued being pathetic. It Half Moon Slashed another couple times and then switched to basic attacking Tenshi for zero damage. Byakuren kept hitting it with Star Sword Apologetics, whittling away its tiny health bar. Then the Magatama finally killed Tenshi with Razor Wind and brought Byakuren down to critical levels. But Byakuren is FAST, even without her 100% Speed buff, and I got one final turn before the Magatama finished me off. One last Star Sword Apologetics, bringing it down to...
Not quite dead. I literally can see no red in its health bar. The Magatama gets a turn. I hold back tears of frustration. And Half Moon Slash. Byakuren is at 100 health out of nearly 4000. I watch in despair as it gets another turn mere seconds before Byakuren's own turn.
Half. Moon. Slash.
One more Star Sword Apologetics does the trick. Magatama defeated, with only one party member left standing.
Labyrinth of Touhou 2
Even more impressive own: I remembered how to spell "Magatama" this time