In chapter 4 right after you leave the Bacchus bar there is a alleyway if you make a u-turn instead of going towards the millennium tower or right. A cutscene of a homeless guy will start, ignore him, take a left and continue north on pink street north.
A cutsceen will start watch it until a choice is asked and go with the girl for a drink. When you are asked if you want a drink, decline 3 times. Defeat the men who appear, get your free 100,000 yen, and on the next choice refuse to let them off the hook. Once they tell you about the hidden casino and leave the sub story end and the achievement will pop.