This isn’t exactly a guide, but the other person that commented said the achievement is bugged. It’s true that it doesn’t work after you beat the game and select chapters. You have to start the entire game over again to do the CATS quest. Every time you go to Rengkok South, talk to the girl, find the cats and then talk to her again before you leave to your next mission. I believe the third time you return to the city, you will get the achievement after finding them and talking to her.