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 side story "Roommates".
Complete the side story "Rekindled".
Complete the side story "Blooming Flower".
Complete the side story "Storm on Your Peace".
Complete Orino's story.
Complete Shiori's story.
Complete Kiku's story.
Complete Atsuko's story.
Complete Nanami's story.
Complete Kiyomi's story.
Complete Izumi's story.
Complete the Prologue.