Mundo Cani challenging Wyrm to fight and then diving into the pit. And then single-handedly driving back the Big Bad of the entire book, Wyrm, with only a cow's horn.
John Wesley Weasel during the battle with the basilisks, after the Wee Widow Mouse is killed. He kills thousands of basilisks by himself, fueled by pure rage, and it takes nothing less than the intervention of The Dragon to stop him.
Chauntecleer's battle with Cockatrice. Especially when he keeps struggling bravely up every time Cockatrice strikes him.
Chauntecleer's last battle against the wolves in The Book of Sorrows.