A Force of Four has Power Girl, the world's greatest and mightiest hero, Huntress -the vigilante daughter of Batman and Catwoman-, the Amazon warriors Wonder Woman and her daughter Fury...
Atonement has Tether. Don't let her cutesy appearance fool you- Mads is an incredibly strong fighter, in no small part due to her large amounts of experience fighting a variety of villains, despite her relative inexperience in comparison with more "capable" heroes.
In chapter 3 an Eldritch Abomination was about to stomp on the team. Asuka showed up and single-handily pushed it back. She fought with such a fierceness Shinji got stunned. And in chapter 7 she came up with strategies to defeat the Monster of the Week.
In chapter 7 Rei destroyed a giant alien monster on her own.
Children of the Atom has lots of these: Sarah from the Modus Operandi arcs certinly counts. Cass and Veronica from the Pro Posturas arcs as well. Jess is an odd case, as she is both the main character of Pro Posturas and not an Action Girl of any kind, instead a Ambadassador.
Daughter of the Sun has female warriors, just like in the source material, but one of the most notable would be Skypaw. She's able to fight a bear on her own (with the help of her special powers, however).
The Star Wars Rebels fanfiction Ezra Lost has Hera Syndulla and Sabine Wren, just as they are in the canon series. Hera shoots it up from the skies in the Ghost while Sabine is typically a gunner in the ship or blowing stuff up/shooting Imperials on the ground.
Tali'Zorah in For Tomorrow We Die has become one of these after her long service on the Normandy.
While there are outstanding examples of this in its sourcematerial, Forum of Thrones has a larger number of prominent female fighters that are portrayed as being equally skilled as their male counterparts, despite the traditional role of women in Westerosi society. Notable examples include:
Samantha Ducard, the dualaxe-wielding sellsword from John Gutten's group and one of the primary Point-of-View characters.
Her friend Sasha, who sports an exotic, eastern fighting style and wields her arakh with dangerous skill.
Janae, another sellsword from John Gutten's group, is a former fighting pit slave and a deadly archer.
It is not unheard that women decide to be soldiers or guardsmen in the continuity of Forum of Thrones. These women, while vastly outnumbered by their male companions, are treated equally by most. Examples for this are Nora Recton and Edonia Bernile.
Similarly, there is a vast number of female characters that decide to join a sellsword group for various reasons and they are typically presented as similarly skilled than male sellswords around them.
Special mention goes to Argella Durrandon, who manages to be this trope, while still fitting into her role as the princess of the Kingdom of the Stormlands. While being fully capable of being a sweet and charming diplomat, she puts special emphasis on the fact that she is not only the princess of the Stormlands, but also the heiress to the kingdom.
Flutterspy: All of Ms. Worthy's ninjas, Fleur, and Fluttershy.
Pretty much every named female hero in Future Shock, but special points go to Kara, and Susan Vasquez who is an absolute badass even before she gains powers of her own.
The female Assassins named but so far not developed by Terry Pratchett in the Discworld. Alice Band, Johanna Smith-Rhodes, Jocasta Wiggs, Joan Sanderson-Reeves, and others, all capable of bringing a woman's gentle touch to the deadliest profession. A good starting point is The Graduation Class.
In The Great Disney Adventure Saga, Kelsey definitely qualifies as this as she finds a will to swordfight whenever she can. Elizabeth is a more conventional choice, and Mulan falls under this category as well.
A Growing Affection upgrades Hinata into one. Yugao might have been one before, but she definitely get to show her chops here. OC Kaede Mizuno also qualifies.
Hermione from Harry Potter and the Methods of Rationality is one, being one of the three generals of the student armies. She's also founded an organization, S.P.H.E.W, the Society for the Promotion of Heroic Equality for Witches, to help other girls become this.
Essentially, the entirety of female heroes of The DCU make at least one appearance in Hellsister Trilogy. You can find flying bricks (Wonder Woman, Supergirl, Power Girl, Mary Marvel...), vigilantes (Batgirl, Huntress...), witches (Zatanna...), telepaths (Saturn Girl...), and more.
HERZ: Misato and Asuka seldom fight the enemy directly nowadays, but they are still duly trained soldiers. Rei also can hold her own since she is a pilot and the merge of two Eldritch Abominations.
In Horseshoes and Hand Grenades and most of its sideories, many of the female characters have been upgraded to fighters. This ranges from Yuki Jojima using her Hayabusa Plushie to punch people into oblivion, Tomoko Nozama using her snake skin as a whip and minor characters like Rumi Egawa charging toward people and kicking them in the face.
Letters From Tomorrow: Chloe Price. While in the original game she was still the most badass person in the main cast, that was a very low bar to clear. In the fic, she's been exercising avidly for years and has been getting serious combat and shooting lessons from her step-father. She has a reputation around Blackwell as the ultimate badass because she easily beat up two star football players, a reputation she proves correct in every fight we see. It turns out that future!Max used her Mental Time Travel to manipulate Chloe towards getting stronger her entire life (including improving her relationship with her step-father so she'd have a teacher) solely so that when present!Max started investigating the conspiracy, she'd have a bodyguard.
Like Father Like Daughter has Seraph, Marcos daughter, who shows the same proficiency in fighting as her father. Both Star and Pony Head remark on how it should be obvious considering her parents.
While her female friends are also considered this, Keena Soga in Keena & The Defendants of Constan Academy is a notable version of this trope. She doesn't need guns or even magic, all she needs are her firsts and kicks and can take as many punches as possible. Remember, she previously was a damsel, and really wanted to prove herself. It took only two chapters for her ass kicking to show up.
Kara of Rokyn has Kara, a super-hero, wrestler and martial artist who still kicks butts every so often even though she is semi-retired.
In Kingdom Hearts 3: Final Stand, Kairi has developed into one. Case in point: in chapter 14, when one of Xehanort's henchmen (revealed several chapters later to be the Riku Replica) tries to nab her, she nonchalantly breaks free of his grip and flings him off of a cliff.
In the semi-genderbent Sword Art Online fanfiction story The Kirita Chronicles, the main character Kirigaya Kazuta (the genderbent version of the canon protagonist Kirito) is indeed an action girl.
While all the girls from the canon Naruto series are capable fighters already, they qualify even more so in Kitsune no Ken: Fist of the Fox, with Tenten in particular playing it straight in that she constantly trains and goes out to fight so as to become stronger.
Asuka's job consists of piloting giant robots and killing Eldritch Abominations that shrugs nnukes off. Later she becomes a super-heroine: Wonder Woman in the original story, Supergirl in the rewrite. As a result she kicks even more butts with her bare hands.
Misato and Rei also are perfectly capable to fend for themselves in a fight. Naoko Akagi also gets to be one when she gets a spiffy new body.
Legacy being an adventure fic starring three female leads and a female antagonist naturally has this.
Rainbow Dash in Fanfic/Legionnaire is a member of the Equestrian Legion, and is a highly competent soldier.
Origin Story: Alex Harris doesn't hesitate in the least to rush in and help when people need help, nor does she shy away from fighting when a fight is what is needed. But then, half of her mental makeup is Xander Harris, and the other half is Kara Zor-L, so its to be expected.
In Paper Mario The Temple Of The Sun, the three girls in Mario's party - Hanner, Dulia, and Maizee - are all this. Fitting, since the first two Paper Mario games always included female partners along with male ones to join Mario. This is no exception.
Every female Power Ranger in Of Love and Bunnies. The standouts are Trini (who scared an entire squadron of Red Rangers into fleeing after they fought Serpentera without her), Kim (who fired an arrow into a fight between the Red and White Rangers), and the girl from the Chinese restaurant (who takes on a swarm of Mooks).
Rick and Morty New Drama Adventures: After her first adventure with Rick and Morty, Kitty begins regularly joining the two on their whacky adventures. Rick at first doubted her abilities, but Kitty quickly proved that she could hold her own and shoot just as good as the rest of them. Kitty isnt afraid to face off against terrifying alien creatures if it means keeping her friends alive (and getting that great selfie)!
The Second Try: Asuka is a better action girl than her canon Ace Pilot self because she's now more experienced and more motivated than ever. May God have mercy upon your soul if you threaten her family because she has bad temper, a giant robot, and slays giant monsters for a living.
Thousand Shinji has Asuka and Rei. In addition to be Humongous Mecha pilots with many years worth of training, the former is a berserker super-soldier and the latter is a plaguemaster with super-healing.
Total Shuffled Island Series has Jo, who saves the day with a doorstop, and is given the nickname "Action Girl" by the Celebrity Manhunt hosts.
Ultimate Spider-Woman: Mary Jane Watson loves to keep up with the latest fashion trends, is a Twilight junkie and enjoys being the centre of male attention at parties. She's also won dozens of fights as a costumed superheroine, with a Rogues Gallery as long as her arm.
Weasley Girl stars a Gender Flipped version of Ron Weasley, Veronica "Ronnie" Weasley. She's just as as hotheaded as her male canon counterpart, but more impulsive and even more likely to charge in where angels fear to tread.
The World of the Creatures: One of the character is Buffy Summers. With seven years of experience, plus supernatural slayer strength, Buffy isn't afraid to go after a troop of cybermen with nothing but her bare hands. The other female characters get shades of this as the story goes on.
Wreckstuck: Zillah counts as this, as she manages to kill a dragon by beating it's head into a bloody pulp.
Coreline: It probably would be easier to count the women that have appeared on stories in this setting that don't have a capacity to fight. Anything from trained soldiers to superheroines, and most that weren't badass within their canon Took a Level in Badass.