Ravenlok

To get in the Mask Mansion, you need to use the Opera Binoculars on the Mask Mural inside the Victorian House.
You get the Binoculars from Atlus the Pianist after you find his sheet of music, which is on the Witch's House Attic.
The attic entrance is on the left side outside of the Witch's house (not inside as I thought :)

Now, you have to talk to the Soot bunnies leader, then go up the stairs, talk to the Panda bouncer.
Then find 4 pieces of mask disguise around the house, talk again to the leader to he can make the mask.

After that, go back to the main stairs and you will be ambushed by the Harlequins.

Easy fight!
Phew on getting here first! :)

edit: remember to level up, if you are struggling. The witch garden has a dude called Decker whose shop can shine your weapons and level you up. I was level 7 at this stage without grinding, just trying to find out where to go in the game.