The most valuable advice is in the Agranor library. But in short: you need to pay attention to Celica’s clothes, the objects she wears, and diligently master alchemy.
No less useful will be the most trusting friendship with a cute translucent character, who can significantly help with the routine while you take a break from the game and make tea.
Each monster may require its own strategy, but usually the girl should have the highest possible willpower, speed and magic power (arcana).
Some monsters require you to drink some potion beforehand.
If there are several monsters, it is advisable to attack one of them each time.