This is a Missable trophy as you have to choose between this and
*** Spoiler - click to reveal ***After fighting through the Elementary School the second time (with humans or elves), you'll be left with a choice inside the 4th grade classroom. Side with Cartman, or side with Kyle.
For this trophy you must side with Kyle. To do this attack Cartman. This will start a boss battle (sorry no summons available for this one). Throw whatever you have against Cartman. He is immune to stun attacks, but most everything else does apply to him (Bleeding, Burning, Gross, etc.). Once he is down near 25% health, he will try to use his Flamming Fart attack to finish you off. Counter-attack with your fart (mash the button shown) to overpower him and win the fight. When you win this fart battle, the trophy will pop.
If you want a full experience, after being kidnapped by the Elves earlier in the story, choose to switch sides to Kyle and attack the school with the elves - This does NOT affect this trophy as only the boss battle choice determines which trophy you are getting.
Also, note that by setting a manual save right before you make the choice between Cartman or Kyle, you can easily reload and choose to fight Kyle instead; thus preventing you from needing a second playthrough to earn both trophies. You will, however, loose out on the difference in the story where you fight against the elves instead of the humans.