Here are the people you must fart on to get this achievement-
1- Father Maxi - He is sitting on a bench infront of City Hall
2- Mayor McDaniels - She is inside of City Hall behind her desk
3- Principal Victoria - She is inside the South Park Community Center. You will be able to fart on her once you complete the storyline quest "Nonconformist" where you will learn a new fart from Randy and will enter the Community Center.
4- The Prime Minister of Montreal - He is located in the caves in Canada. To get to him you must first learn the Nagasaki fart from Terrance & Phillip to destory the rock. When you get next to him, fart on him first before talking to him as he will leave the game forever when you talk to him. He IS missable. It's worth noting that you are required to talk to the Minister during the storyline quest "O Canada" at the end of the quest.
I reccomend using every type of fart on each person just to make sure.