The mansion is one of the two locations where you can get the disguise for Aiko in Nagoya (the other location has two naked male civillians getting massaged). There's a rock in the courtyard that you can use for "Good Old Rock...Nothing Beats That". There's also a flower pot on a balcony outside for "Sunflower Samura".
If you have killed the enemies at the mansion, the achievement will pop up once you complete the level.