Sara. Admittedly, she looks like the Faux Action Girl for most of the first season, and is quickly defeated in the first battle she has, but the first battle of hers that the viewer actually sees is one of the most intense combats in the series up to that point.
Some female creatures on the show also provide examples, such as Intress, Laarina, and Takinom.
Gravity Falls: Mabel, Candy, and Grenda have all displayed definite Action Girl traits.
"Into the Bunker" proves that Wendy is most definitely one of these. Pacifica also handled herself petty well in "The Golf War" and further cements her action girl status in "Northwest Mansion Mystery".
Ninjago brings us Nya, who takes down a horde of skeleton warriors on her own in one of the mini movies. She then creates a samurai alter ego and even surpasses her male teammates in fighting at some points. It took them ''forever'' to figure it out. She learns in season 6 that she possesses the elemental power of water and trains to be a ninja alongside the rest of the team.
Most of the major female characters on Phineas and Ferb get a chance to play Action Girl at some point or another. Recurring character Vanessa may be the best example, though Candace, Isabella and the other Fireside Girls get plenty too. Candace once wrestled an alligator for an Achievement Patch. The very fact that this was an Achievement patch for preteen girls speaks volumes about the Fireside Girls.
From ReBoot, all of the main female characters were more than capable of holding their own, though AndrAIa was the only one to be consistently in the fight. Dot and Mouse were more often used for their command and technical skills.
Samurai Jack: The Scotsman's Wife. She singlehandedly beats an entire army of demons that both her husband and Jack - the two most badass warriors ever - had trouble fighting. She starts off the fighting by punching a Celtic demon god in the gut because said god called her fat. She's also the only person Jack has ever, without question, ran away from a fight with.
Likewise from the final season, Ashi after her Heel–Face Turn. Dear God, Ashi. Just watch her jawdropping battle with the Orc Army or her fight against the Dominator for a demonstration.
She-Ra: Princess of Power has the title character, as well as Huntara, a purple-skinned Grace Jones lookalike who has no powers except a bunch of really cool weapons, but held her own against She-Ra in a one-on-one fight. Other girls count too. The guys were usually Dudes in Distress which is understandable since the show was created as the girl's version of He-Man.
The tomboyish Dahlia and superpowered Lana of Skyland.
Sam, Alex, and Clover of Totally Spies! seem to take turns being competent.
Sam often leads the team, displays the best hand-to-hand combat skills and gets kidnapped least. Clover combines this with Action Fashionista. Alex usually serves as the heavy hitter, and is also the most athletic of the three.
Transformers has a pretty good record with this, despite females being in the extreme minority.
Transformers Animated: Blackarachnia, Strika, Starscream's female clone and Sari, after she took a level in badass. Professor Princess might technically count, although she's more about the gadgets than actual combat. Arcee might have been one, but never got the chance.
Transformers Prime: Arcee gets the chance. And she runs with it, cutting a swath through Vehicons and never backing down, she's a seasoned, battle-scarred warrior.
Miko the human gets a few action moments, including one episode wherein she gains possession of the Apex Armor.