Abyssal Renko (Labyrinth of Touhou 2 Plus Disk)
Due to Abyssal Renko's extremely random normal moveset (which includes moves like Half Moon Slash, Refresh, Verbal Abuse, an attack that inflicts SIL, and Fluster, a turn wasting move unique to all versions of Renko), it feels best to treat this fight as a damage race. Note that you don't want to advance this fight too quickly because Renko can use her annoying "Ability to Hide the Stars" and "Ability to Hide the Moon" attacks when you've hit certain HP thresholds.
These two moves (which share the same animation as her personal skill, Galaxy Stop) will inflict a barrage of ailments on your entire front row. Hidden Stars can inflict PSN, PAR, SHK and HVY while Hidden Moon can inflict SIL, TRR and DTH. The first two uses of Stars and Moon (at 85% and 70% HP respectively) will be underpowered, while all the other uses will be much stronger. Abyssal Renko retaliates with these attacks about six times in total, but it's totally possible to kill Abyssal Renko before she can use it six times if you take the fight seriously (instead of playing with your food, like I did here).
If you have Potion of Life item, use them. While they won't protect from every ailment, they can protect from most of them (including DTH). As far as offense goes, you're honestly spoiled for choice here. Her weaknesses to both MYS and PHYS mean you've got a lot of possible attacking options here.
Abyssal Renko is slightly resistant to the four natural affinities, but is weak to MYS, DRK, SPI and PHYS. On paper, the only ailment you can normally land on Renko is debuffs. But since her Heavenly counterpart is weak to DTH, it might be possible to inflict DTH on the Abyssal version too.
Music is "Strange Bird of the Moon, Illusion of Mysterious Cat" from Ghostly Field Club.
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