The Greenhouse is accessible from Floors 3-5.
It's a tiny branch and it doesn't lead anywhere. The most notable thing about this branch is that, during the day, it's flooded with sunlight. Who knew? The sunlight poses an existential threat to vampires, especially if caught unaware during sunrise.
The Greenhouse is filled with Vipers, Toadstools, and Green Men. Green Men are very powerful when drenched in sunlight.
During the day, the Varcolac is in his human form. But he is cursed to turn into a werewolf at night and in this form he is much stronger.
The question is: what form should you be to confront him? Ideally you would fight him when he's weaker, but you'd have to be vampire for that. Since the Varcolac is only human when the sun is out, you risk getting a serious sun burn. One option is to turn into a werewolf yourself if you have the ability to do so. In any case, this will probably be one of the easier boss battles. Do not try to fight the Varcolac during a full moon!