A mod over at Toho Kingdom writes yearly holiday themed fanfics with members of the site as character. Last Christmas fic featured Cthulhu as the main villain. After the military fails epically to kill him, they get a pair of mecha from Santa Claus and manage to kill him.
In Dumbledores Army and the Year of Darkness, Ernie Macmillan has the stones to punch out a Dementor. Yeah, he goes down...but he tried. And he gave his secret wife, Susan Bones Macmillan time to whip out her wand and summon her Patronus.
During his first appearence in The Tamers Forever Series Chaosgallantmon absolutely DESTROYS both Azulongmon and Zuqhaiomon in about ten seconds flat.
In Manchester Lost, Crowley punched Lucifer - In the FACE!!! There's also the part where War yells, "Mommy, you're an idiot!" followed by decapitation.
In My Immortal, it is the reader. This really isn't much of a surprise, giving its reputation, but screw it. I wanted to test the spoiler cover's powers.
The Spore wiki fiction universe has a rather epic one involving the hero fighting and actually defeating Satan(A.K.A. Kamik'Shi) and then sealing him in the sword that stabbed him.
In Soul Chess, Lelouch did what Ichigo in Bleach could only DREAM of and Geassed Aizen to kill himself. It's considered by many as Lelouch's greatest achievement in the fic.
