Here is the full list of all 50 South Park: The Stick of Truth achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
You have collected half of the available costume sets.
You befriended your father.
You defeated Princess Kenny and recovered the Stick of Truth.
You've spent $500.
You made friends with half of South Park.
You opened all of the garages in South Park.
You watched your parents have sex for 60 seconds.
You defeated the High Jew Elf Kyle in battle.
You befriended the Goths, like everyone else playing this game.
You collected 15 Chinpokomon.
You sold 300 junk items.
You farted on 10 enemies in the world that were knocked out.
You've farted on people 100 times.
You completed your first day in South Park.
You have joined the KKK.
You've unlocked all of the upgrades for a New Kid ability.
Over the course of the game, you've scored 100 perfect attacks.
You flung a turd of your own creation at an enemy.
You successfully blocked every enemy attack in a single (non-tutorial) battle.
Over the course of the game, you've blocked 100 attacks.
You collected 5 Chinpokomon.
You defeated 3 enemies in one battle while your buddy was knocked out.
You defeated the Grand Wizard Cartman in battle.
Gingers have bitten you 3 times without you blocking.
You defeated the Boss Hall Monitor while wearing freckles.
While inside Mr. Slave, you summoned Mr. Slave.
You found Jesus while playing as a Jew.
You completed the game without selling any items.
You defeated Princess Kenny while looking like David Hasselhoff.
You farted on four major authority figures.
You shat your pants during a boss battle.
You accepted your fate and chose the name Douchebag yourself.
You've farted on a dog while in gnome form.
You knocked out 3 or more enemies with a single fart outside of battle.
You defeated Princess Kenny in your girl makeover disguise.
You've farted on every buddy in the game.
You interrupted 5 channel attacks by using a fart.
You defeated the Meth Tweekers while wearing the Evil Cartman goatee and bald cap.
You defeated Princess Kenny while living with Dire AIDS.
You completed the game and never ended a battle with your buddy knocked out.
You collected all of the Chinpokomon.
Everyone in South Park is your friend.
You defeated 20 enemies using Cartman's Ass.
You've farted on animals 25 times.
In three separate battles, you've taken a shit in your pants.
You've collected all of the available costumes.
You farted on the corpse of an aborted Nazi Zombie fetus.
You befriended both crab people and gnomes.
You've let Kenny die in combat 10 times over the course of the game.
You own all the weapons and costumes available in the game.