In the First Book, The Stone. Galen spikes the potion to get through the fire after everyone had left, knowing Quirrel would come to the right conclusion to the riddle and drink it, but not knowing its been spiked and thus useless. He burns.
Shirou puts down the Basilisk by transfiguring a large rock into a bow and using the Sword of Griffindor as an arrow.
He does a repeat performance during the siege of hogwarts in Book 6 on Nagini.
Voldemort's assault on all of the Scarlet Seven's homes being countered, especially Ginny coming into her own as a strategist.
Kiritsugu ambushing and killing almost every Death Eater Voldermort had with him in his assault on the ministry for the prophecy by himself armed with a broom, an invisibility cloak, and a bolt action rifle using basic military tactics. Voldemort only survives due to his shot hitting Kirigutsu's fingers causing him to shoot him in the knee instead of the heart.
Kiritsugu resuming his job as the Magus Killer and assassinating Voldemort's allies one by one.
In Book 6, Chapter 46 Shirou recreates a variant of Unlimited Blade Works.
Tracy Davis gets credit for managing to subvert Hogwart's defenses for the siege. It failed, ultimately, but she came closer than Nott and Malfroy by far.
Amelia Bones last stand in Book 7, Chapter 7.
Book 7, Chapter 31 Gallen versus Mind-controlled Hermione and Feral Vampire. He wins.