Magical Girl Hunters

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Magical Girl Hunters is a dark comedy Round Robin story originally created on Improfanfic. It follows the exploits of a pair of sociopaths, Yoi and Itami, who have established a business killing Magical Girls. Within the context of the story, Magical Girls are being produced at a prodigious rate, to the point where they're running out of original concepts and proper evil enemies. Therefore, some magical girls have decided to branch out into more broad social evils such as polluting corporations. These corporations, unhappy to have their factories trashed by magical forces of goodness occasionally hire Yoi and Itami to rid them of their problems. Occasionally dark forces also hire Yoi and Itami. Some of the dark forces also support their own teams of magical girls... by the end of the story, it's become a grand battle of good and evil and Yoi and Itami are in it for more than just the money.

Tropes used in Magical Girl Hunters include:

Air Vent Passageway: Yoi's entry and escape into the Mashihaishi Ultra building. He gets lost a few times, but since he runs into others also using the vents (at least one of which has been crawling in them for several years and was engaged to kill the prior owner of the complex), he's able to get directions.
Child Soldier: Almost all magical girls are treated as this by whoever empowered them. And, just like with real-life recruiting, there's a portion of them in their to fight for their ideal and a portion that just likes the license to kill things...
Dark Magical Girl: Multiple cases. Pretty Deadly and Sailor H are both cases of magical girls who felt abandoned and have thus turned to killing magical girls themselves.
Eldritch Abomination: The Sailor Shoggoths
Enemy Within: Yoi has Koi who starts out as a typo in one of the chapters, turns into a split personality, and eventually becomes an independent entity.
Enfante Terrible: Captain Kawaii... *shudder*
Evil Weapon: Itami wields one of these. When the first one breaks, he acquires another one in short order.
Gender Bender: Early in the series, Yoi gets turned into a Magical Girl temporarily. One of the "fixes" they acquire makes it permanent. Mai and Kyo provide another example. One is male and one is female. They've forgotten which is which because they tend to switch every time they die.
Genre Savvy: One of the only ways Yoi and Itami have survived taking on the magical girls is their ability to exploit the aspects of the Magical Girl genre.
Heroic Sociopath: Both Yoi and Itami could be considered this. Or possibly Villain Protagonists. They slaughter little girls for money, Itami is a stone-cold killer who's been mentioned to have "played doctor" back in school with real knives, and Yoi is unawares the embodiment of chaos and entropy.
Incredibly Obvious Bomb: In one of the chapters, Yoi comes back to find a bomb taped to the doorway of the office. In plain view.
Killed Mid-Sentence: The very first scene begins with a Magical Girl making her dramatic entrance, complete with song, and getting headshotted before she can finish the first verse.
Knife Nut: Both of the protagonists carry knives because "knives don't jam or run out of ammo" but Itami stands out for preferring to use his knife or sword on the girls they hunt.
Knight Templar: Due to niche overcrowding, there are magical girls devoted to stopping littering or illegal software downloads. With deadly force if necessary.
May-December Romance: Itami/Sailor H, Yoi/Aika.
More Dakka: The other major reason Yoi and Itami survive their encounters with magical girls is that they have no fear of bringing in heavy weaponry.
Neck Snap: Used for the second kill in the series. One-handed. With a Neck Lift. Of course, the target is also an 8-year-old girl...
Of Corpse He's Alive: Pretty Marshall Rose's body is used to sneak their way through a complex.
Paper-Thin Disguise: After Yoi steals money from Mashihaishi Ultra, he's visited by a very badly disguised youma.

It stood there, sorta lopsided, dressed in a trenchcoat... it had a black wig over its head, and under that I could see a cheap plastic mask, of which the lower jaw area had been cut free so the “mouth” could appear to move. The best part, however, was the sloppily-lettered bit of cardboard hanging from its shoulders by a strand of twine, that read, “Me Itami.”

Pink Mist: Frequent descriptor for headshots on the magical girls.
Time Stands Still: The duo makes good use of an unexpected time-freeze to dispose of the Crayola Knights. Given that there were sixty-four of them and they were introduced as a very nasty cliffhanger in the previous chapter ("Screw Your Neighbour" was an established policy in the writing at the time), the alternatives would have been less than pleasant.
Tome of Eldritch Lore: The Necronomicute