This is a story related achievement which will unlock with natural story progression.
You will need to spray Big Danny with the perfume that Bella gives you. Once you have done that, head over to the pet store and talk to the shop owner. He will tell you Bert is well again, your next step will be to buy all the kebabs from the guy on the street.
Once you have the food and collected the cats, Big Danny will cry out while the cats jump all over her. At this point the police will leave the station which allows you to sneak in undetected. The achievement will unlock as soon as you enter.