There's plenty, but Cornelius getting up after being contemptuously blasted aside by King Valentine and then "bitch-swording" the smirk off his face is near the top.
Mercedes's fight with Demon Lord Odin. This was the real deal, no Demonic Possession, and he had an Infinity+1 Sword on his side. After starting as a brat, Mercedes stands up for the sake of her kingdom, defeats Odin, and breaks the Balor, putting an end to the conflict.
What happens after the battle is arguably even more badass: everyone, including Odin himself, clearly expects Mercedes to kill him and avenge her mother's death. Instead, she declares that would be too easy - it would only make him a martyr, and his successor would pick up where he left off. She delivers an ultimatum: if he promises never to attack Ringford again, she'll spare his life. If he chooses to die rather than make that promise, however, the fairies will attack Ragnanival after his death. Not only does she defeat the Demon Lord in combat, she completely shames him in a brilliant political move that firmly establishes her as a worthy queen and a force to be reckoned with.
Gwendolyn performs increasingly insane feats throughout her story, but the last three count.
Staring down Leventhan certainly counts, much less bitchslapping the whelp where Oswald could not defeat him. Read that again - the Shadow Knight was thoroughly wasted by a whelp that would grow into one of the Five Disasters of Armageddon, and then this valkyrie dives in and smacks it dead.
"Your Majesty, if you persist in insulting my husband, my spear will take your hand."
Next chapter, launching a one-woman assault on Ringford Forest, all the way into the Fairy Queen's own castle, to request the return of the Ring Titrel. When Queen Mercedes refuses to relinquish the ring, Gwendolyn kicks her ass in her own castle and takes it back anyway.
...only to turn around and dive straight into the Netherworld and cut a bloody swath straight to Oswald, up to and including the fucking Queen of the Dead! All out of love for him.
And on a more personal level, she follows these feats up by facing down her father, whose approval and affection she has desperately and fruitlessly pursued for a good bit of her book, and flatly refusing to give him back the ring Titrel because It Was a Gift from Oswald - someone who, unlike Odin, respects and values her as a person and is willing and able to give her the love that Odin won't.
Mercedes's fight with Onyx, single-handedly avenging the burning down of her entire forest. Doubles as a Tear Jerker.
All the cool stuff Ingway is able to do as a friggin' FROG certainly qualifies.
As soon as he casts the Pooka curse on Beldor in the Titanian sewers, you knew that it was gonna be awesome.
The Prelude of the Armageddon book: Ingway, still under the influence of Darkova and near death from his last battle, stands up to his Magnificent Bastard grandfather, King Valentine, and threatens to use the power of the Darkova form to stop him once and for all. When asked by the king if he will fight to the death, he replies with one immortal line - "Aye, unpleasant, isn't it? The cowardly prince who destroyed his country will give his life to save it." - and then proceeds with said terrible transformation.
Another brief moment from Ingway: completely wrecked after his second transformation, he nevertheless finds it in him to stab Beldor in the back when the latter tries to throttle the location of the ring of Titrel out of Mercedes. "You control me no longer."
Make the correct character decision and enjoy watching Oswald casually and fearlessly approach King Gallon in preparation for the upcoming epic battle.
One final one after achieving the good ending and defeating the final boss: As Gwendolyn is falling to her death, having lost her ability to fly, Shadow Oswald pulls a Big Damn Hero moment to leap who knows how high into the air and catch her.