As an old but new player here, I also referred to all these and other threads to the game...now done a own compilation and guides on those H-scenes and some miscellaneous here
This series basically you can characterized your own girl and lots of stuff, but the H scenes are separated to 4 groups of different characters and slightly variable among themselves. The 2 protagonist have more scenes and storyline,and you don't need to play along the quest or clear dungeons to get all the H scenes (except few for that 2 main protagonists)
Basically how to do it is all below
First Night - Become couple
Second night - Invite to bed again next day
Anal - Buy an Anal book (cost 30000 maybe) and equipped in main character
Outside H - success on Short Chat Session (different characteristics have different goals, depends on the girl)
Masturbation - let the girl in party get aroused, then talk to her and sleep overnight, by either tentacle on 1F right map or buy from mysterious guy. Pink dildo needed to which you can get from mini game or brought..second one is the same for protagonists
Masturbation 3 - Take the specific quest and catch the watchman dog from the map of 3 statues, on the right side. Thats the room with Dogmaster...before passing the quest follows the same step as above
Underground 1-5 - Bankrupt in casino, let the girl stay and visit underground everyday...remember the girl need to become couple and a high Love level or she won't follow or stuck in 2nd or 3rd scene
Tentacles - Fight with tentacles in 1F right map on upper comer...let the girl at least be hit 3 times then lost...but the girl with most hit always become the victim. you can find her on the hidden corner in the same 1F but further. Fight the Tentacles king to get her released.
Two special one with Refia and Serusen one is underground another one is beat her 1vs1, then choose rape instead of kill
I recommend you do a separate save if you want to see the H scence of underground and tentacles, cuz her Love level will drop real fast and she is dead inside after and won't talk with you anymore
The Short Chat session one is quite hard and you need to memories some sequence which they like.
I am currently at 5F now so anymore then that I can't tell but welcome discussion