If anyone else is wondering about farming, I tested some stuff out and it seems like the best way to farm is re-fighting bosses. Each clear, you get a guaranteed drop from the chest, and you can get either a Nice Clear or No Damage bonus that boosts XP and I think loot as well. No hit seems like around 5x XP gain, and guaranteed red chest(the best materials for that point in the game, like sturdy fur against the ice woman but tough fur for the Plant girl), while Nice Clear might be based on time taken, and I think is x2 XP gain.
Otherwise, and for the late-late game stuff like Spirit stones in the demon castle, the best method is finding a room with a pit. Falling in a pit respawns you and resets the room, but you keep items, XP, gems, ect. That means kill the girls, fall in the pit, and not only do you get full HP and SP back, you also start right back at the room entrance. And a reminder, vertical rooms are also great for it, as if the bottom exit is a ladder, falling without using a ladder counts as a pit, so you can easily respawn that way too.
The best spot I've found is the demon castle, go up from the first teleporter and follow until you reach the small room with the giant swimsuit girl. Get ice, jump spam your special, and drop down the hole to reset. I had all the drop rate boosters, and she dropped Obsidian every so often, and I got one spirit stone drop, but couldn't get anything else. For other items, get to the second massive room before the Demon queen. There's a pit by the elevator, or just get an assload of SP restore, and using Ice, you can clear the room relatively easily. The flying girls don't seem to drop anything at all, ever, the wolf girl dropped a forbidden scroll, and the other three girls dropped spirit stones and sunlight stones