Persona 3 Reload (EU)

After completing all of the floors in the second door in the main desert area you will get to a point where you will witness a cutscene of Ken speaking with the police after his mother's passing in the past, the achievement will unlock at that time. The battle before this is rather long but dropping the difficulty to the lowest setting will make it a breeze and does not affect the achievement. This is not missable and will be the first story-related achievement you unlock in the DLC.