Sunken Fossil World/Flandre Scarlet (C)

Note:
Flandre's routes are a bit unique compared to the others.
The playable characters all head to the Underworld, save for Yuuma who already lives in Hell. Meanwhile, Flandre is at the Scarlet Devil Mansion, her home, where Okina greets her and uses her powers to generate three doors for Flandre to walk through.
Each of these doors sends her to a different enemy, explaining the split between Routes A, B and C. Therefore, unlike other routes, only the introduction cutscene is named, and its chapter is called The Secret God and the Destructive God's Long Day.
For the sake of clarity, the format here will be Stage # - Location.

Now, pick any door you please! – That door pits you against a strong foe. You'd best prepare yourself for a tough time if you choose to take it on. In fact, she might even be stronger than you if she's serious.

—Okina Matara

Stage 1 — Old Hell Hot Springs

Versus Yuugi Hoshiguma

Stage 2 — Underground Fusion Reactor

Versus Minamitsu Murasa

Stage 3 — Former Blazing Hell

Versus Kutaka Niwatari

Stage 4 — Sea of Petroleum

Versus Yuuma Toutetsu