At the end of the second day, you have to choose to choose between Cartman or Kyle. Depending of you choice, you will have a duel boss against either of them. At the end of the duel, you will have a Fart O War between either of them.
The final boss fight with Princess Kenny. When Kenny uses his special abilities (summoning his Nazi zombie unicorn, Nazi zombie rats, Nazi zombie Death, and drink more of the green goo), your party members would prevent him from succeeding. Stan would decapitate his Nazi zombie unicorn, Jimmy would play his pipe to lure the Nazi zombie rats away, Butters would transform into Professor Chaos and blast Nazi zombie Death away, and Kyle would kick Ike towards Kenny to stop him from drinking more green goo.