1. As far as I know the door your referring to cannot be opened in the current edition of the game. The actual boss is the giantess with boots which is south of that door. You fight her, get your final party member, and then move on to the next area.
2. Without knowing how far you are into the game it is hard to know exactly what you need to do next but I suspect the NPC is talking about the giantess in the sewer system beneath the monster town/mansion. You need to hit a lever to lower the water level in the sewer system and then there is a long corridor with a boss fight.
The last few months of updates have been very minor so this game has not got a lot of attention. I believe we will *not* be getting an update this month either, but I am basing this off a google translated statement from the author so I could be wrong!