Advice for the latter parts of the game:
Recollection Room:
Unlocks once you beat the main storyline. Accessed via the crystal ball in your inn room (Accessible both night and day).
Sub-Events:
There are events for Liese (elf girl), Kuu (blonde twintails), and Ellen (church nun). Kuu's and Ellen's events unlock via story progression, while Liese's unlock each time you defeat one of the four Guardians at the Trial Temple.
Kuu's and Liese's events are finished once you unlock Free-H mode with them, while Ellen's finish once she gives you a really nice piece of armour (you unlock Free-H mode with her before this particular event).
During the ending, extra/extended scenes will occur for each of the sub-events you complete.
Free-H mode just means you can sex them up however much you want at night.
Trial Temple:
Unlocked via talking to Liese at night early on.
The first 3 guardians aren't particularly difficult if you're decently levelled with good gear; the only thing that really stood out for me is that the 2nd one will cure itself of debuffs, so there's no point trying to utilize poison, blind etc.
The fourth is a whole lot more difficult. Not only is it much stronger, it uses two buffs alternately; the first reflects physical attacks, and when that wears off, the second reflects magic attacks, and then repeats.
The final fight is the Violet Dragon. First off, note that you do NOT have to beat it for Liese sub-events, only the first 4 are necessary. Second: this thing is stupid strong and will easily waste you even at max level (50); it's much stronger than the final boss. You'll need to do some serious Drug buffing to fight it (the Drugs can be bought at the Labelda item shop for 4000G a pop). The Gran Aura skill is incredibly helpful in this fight.