The second half of your trek through the Scarlet Devil Mansion begins with a switch that will open a door leading straight to the area's entrace, allowing you to skip traversing through Voile each time you want to return. This area functions very similarly to the last, in that you'll need to find keys to progress. Your goal is to traverse the left and right sides of the main hall, which will bring you to a room containing two crystal balls, one accessible from each side. Activating both crystal balls will unlock the final hallway in the center, allowing you to progress to the boss.
Boss: Remilia Scarlet and Nitori Kawashiro
Remilia Scarlet's HP: 600 (Regenerates 50 per turn with Dark land active)
Nitori Kawashiro's HP: 500 (Regenerates 50 per turn with Water land active)
A little more dangerous than the boss fights previous, but still relatively simple. Unlike before, the two can actually be moderately threatening. Plus, they're the first bosses you'll fight with HP regeneration, though it is based on the land element, so using the opposite element against them will both be dealing significant damage and preventing their regeneration from taking effect.
Remilia mostly uses physical attacks, occasionally with the dark element. Her most dangerous spell is Red Magic, which has a chance of charming any of your party members, which will really slow you down until it wears off. Trying to eliminate her should be your top priority, but be aware of her significant elemental resistances: everything except Magic, Slash, and Crush damage will be resisted somewhat. The exception is Light damage, which she is weak to, and will prevent her regeneration from kicking in by manipulating the land effect, so use Reimu and Byakuren to take her down fast.
Nitori doesn't pack anything as dangerous as Red Magic, but she does pack magical damage, which is noticably tougher to reduce at this part of the game. To take her down, make use of Mokou's fire-elemental spellcards and Marisa's Hellfire to deal the most damage, and also weaken her water spells and prevent her regeneration. Although, unlike Remilia, she carries only one (significant) resistance to water element, so just about anything can damage her.