In The Wizard in the Shadows, at the final battle of the Black Gate, all seems lost. Harry, the most powerful weapon in the arsenal of the Light, is dead, the Sword of Gryffindor that allowed him to draw on impossible amounts of power is inert, and the forces of good are sorely outnumbered. Then Ginny — up until this point merely quite powerful — picks up the Sword and says a Little "No". The Sword responds. Cue three giant tornadoes of fire and a whole world of hurt being laid on Mordor's forces.
Rise of the Galeforces: Everyone.Just...everyone.Special mention goes to Violet in particular, since her finally being allowed to cut loose and demonstrate to the bad guys what she is truly capable of is pretty much the reason the fanfic itself manages to outlast all the hell it gets put through.
In the Pony POV SeriesDark World timeline, Derpy Hooves has spent a thousand years cursed to walk around Discord's castle until she does it enough times to earn her children's freedom from their own curses, all while being constantly attacked by a murder of blackbirds that are trying to eat Dinky (who's been turned into a muffin). She does this without a single complaint, staying focused on her task... until Twilight and the other redeemed Elements of Harmony convince her to help fight against Discord in order to save Dinky and Sparkler without abiding by Discord's fixed rules. Derpy then proceeds to prove her worth as the new Element of Loyalty by helping to fight the blackbirds, playing a key role in defeating them.
She does it again in the fight against the Valeyard. She spends most of the fight holding back due to the Valeyard being a Discorded Doctor, but when he eventually pushes her too far and she finds out he's actually the Doctor's Super-Powered Evil Side, she then proceeds to go into a Tranquil Fury and beat him to death, ensuring that he'll change back into the Doctor.
This is pretty much a major aspect of her character; when there's not a fight to be done, she's a kindhearted Ditz, but once her friends or children are in trouble, she gets serious and is more than capable of kicking flank.
Back in the main timeline, the Crusaders do this during the Wedding Arc, in response to Sweetie Belle and Spike getting brainwashed — despite just being fillies, they manage to sneak past the Changelings, knock two out, snap Spike out of it, and then cover their escape by summoning a water elemental. Heck, the whole reason the adults let them help anyway is because they can tell by how determined they are to help that they'd do it even without permission anyway, plus, with the amount of stuff they get up to and survive on a regular basis, them taking things seriously makes them an actual threat.
In the Buffy the Vampire Slayer / Power Girl crossover Origin Story, Alex Harris (a Kryptonian trapped in the Marvel Universe) and her domestic partner Louise are attacked by the telepathic assassin Typhoid Mary. Mary knocks down Louise, then turns her attention to Alex and proceeds to use her mental powers (against which Alex has no defenses) to wreck Alex. Turns out ignoring the normal, untrained girlfriend was a mistake when Louise proceeds to clock Typhoid Mary from behind with a bottle of sparkling grape juice, then continues to beat the assassin with it until Mary is unconscious and bleeding on the floor.
At the beginning of The Celestial Renegade in Sailor Moon: Legends of Lightstorm, this is the reason Luna takes Sailor Moon to Lightstorm. Lightstorm causes this to happen with Sailor Moon over the course of a single afternoon. And that was before he came to Tokyo and began training the rest of the Scouts.
In How Trixie (Somehow) Saved Hearth's Warming, Trixie is her normal hammy, bragging self and just manages to skirt by in the situation she's gotten herself into (namely saving Hearth's Warming) by being Mistaken for Badass. However, after being turned into a nutcracker by the Rat King, she goes into a Heroic B.S.O.D. until she finds out Vixen was desperately trying to finally stand out among the rest of Santa's reindeer and is having a Heroic B.S.O.D. as a result of their capture. Trixie quickly realizes this means the two of them are Not So Different, both feeling Overshadowed by Awesome by somepony else and just wanting to stand out. This realization causes Trixie to snap out of it and devote everything she has to getting them through this. She ultimately stages a prison break and defeats the Rat King via quick thinking and plenty of Indy Ploy, saving not only the two of them but all the Rat King's slaves. Even Trixie is surprised.
Havoc does this in the Elemental Chess Trilogy. Just like in Fullmetal Alchemist, he's a bit goofy and loves to make jokes, and has a powerful nostalgic streak. When things go south for his beloved commanding officers, however, he steps up and takes charge, reminding everyone just why he's one of Roy Mustang's most trusted subordinates.
Once More with Feeling: Shinji used to be wimpy and non-confrontational, but after returning to the past, being aware of what will happen if he fails, he is way more dangerous and fights like a berserker.