Slayer of Keraptis
That's because the Monster Manual says that "[t]o kill a troll, the monster must be burned or immersed in acid, any separate pieces being treated in the same fashion or they create a whole again in 3-18 melee rounds."
It never says that you can use "fire" or "flame" ANYWHERE IN THE DESCRIPTION!
By the powers of RAW and RAI, and because I once saw a copy of a monster manual, I demand you stop railroading me into using fire on trolls. Because burned doesn't mean fire, it clearly means, um, freezer burns.
Nothing worse than tyrants trying to end my player agency!!!!!
Don’t you see? Clearly “burned” means to suffer from being the butt of a wicked joke.
“Yo mama so fat, she has to use a boomerang to put on her belt!”
“Way to go Bob, you killed the troll for good! Sick burn!”