The Marry me, Nightmare Add-on for Groove Coaster has 6 achievements.
Play Marry me, Nightmare 20 times
Play Marry me, Nightmare and clear all of SIMPLE, NORMAL and HARD
Play Marry me, Nightmare and get A Rank in SIMPLE, NORMAL and HARD
Play Marry me, Nightmare and get S Rank in SIMPLE, NORMAL and HARD
Play Marry me, Nightmare and get S+ Rank in SIMPLE, NORMAL and HARD
Play Marry me, Nightmare and get S++ Rank in SIMPLE, NORMAL and HARD