Happy Dungeons

This achievement will auto unlock if you've already fulfilled the criteria. If not then simply complete Chapter 8-4 "Front and Back Stage" on any difficulty to unlock it. You will need to beat all previous chapters and missions leading up to this to unlock it. It recommends a level of 27 for Human difficulty so you'll want to be sufficiently leveled or have decent gear to complete it. Playing with other people will make this easier.