Cartman is a party member unlocked about halfway into the main story. What you want to do is select his second ability called "Burning Cloud". This ability will have Cartman set a fart on fire and spay the fire on all enemies in battle. This ability causes a lot of damage too, so you will be using this move regardless to defeat enemies anyway. If you haven't unlocked this by the time you finish all the main quests, simply head into the Lost Forest and keep killing dogs with the ability. Once you defeat 20 enemies, the trophy will unlock.