The opening fight scene of Jake (Heavy) vs. Delilah (Brute). Jake, a massive, muscle-bound hulk of a man, punches Delilah, a petite woman in the face. Delilah just says "You messed up my makeup!" before punching Jake through a concrete wall.
Jake vs. Rokusoburu, who is an indestructible Massive. Sullivan beats him by multiplying gravity over him to fifty times its normal weight and effectively disintegrating him.
Recalled in the third book when he warns Dr. Wells (another Massive): "I've fought Massives before. They squish just like anyone else."
The whole final battle of the first book: pirate-zeppelin and high-speed pleasure craft vs. the Imperial flagship and the most powerful battleship in the world, involving a teleport spamming duel between Faye and Toshiko (who's a ninja) on top of a flaming zeppelin, not to mention a gravity-churning duel between Madi and Sullivan.
Heinrich meets an Imperium Lazarus (an Active who creates zombies) who boasts "my power is suffering!" Heinrich demonstrates the true meaning of the word by phasing the man into (not through) a nearby wall.
Ends with Faye: a small, adorable, poorer-than-dirt Okie girl facing down the Chairman: a seasoned warrior, the most powerful Active on Earth, and the de-facto leader of Imperial Japan. Faye wins. It's not even close.
Faye finds one of the men who helped kill her grandpa. When she asks him about it, he boasts about killing "the stupid Mexican with his stupid brat and his stupid cows..." She then stabs him in the heart:
Faye: "He was Portuguese. And cows. Aren't. Stupid!"
George Bolander facing Crow's Greater Summoned form one-on-one. He uses all his power at once, bringing down a hundred lightning strikes in a second. It kills him, but it kills the demon too.
And then it starts raining where he is for the first time in six years. He single-handedly ended the magic-induced equivalent of the Dust Bowl.
Near the end of Spellbound, Sullivan is trying to interrogate the OCI chief. He doesn't get very far until Faye shows up to play bad cop. The OCI chief's reaction is the definition of Oh, Crap.
Heinrich, deprived of his powers, is confronted by a half-a-dozen large, swaggering OCI thugs after having been imprisoned for several days. They decide to have some fun beating him up. He fights them to a standstill (nearly killing one of them) with nothing but a handful of rusty nails and some very dirty fighting.
At the climax of Warbound, Faye teleports to an Imperium airship and goes to work, finally realizing how to use her true power, and obliterating the vessel. Total time to destroy a warship large enough to mount a Peace Ray: Three minutes, forty-seven seconds.