Here is the full list of all 12 Flowering Nightshade achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Complete the Prologue.
Complete Izumi's story.
Complete Kiyomi's story.
Complete Nanami's story.
Complete Atsuko's story.
Complete Kiku's story.
Complete Shiori's story.
Complete Orino's story.
Complete the side story "Storm on Your Peace".
Complete the side story "Blooming Flower".
Complete the side story "Rekindled".
Complete the side story "Roommates".