The end of the Faudo arc contains one for just about everyone- Ponygon, Kanchome, and Tia all fight bravely against Rodeux despite getting beaten. Momon alerts everyone that Kiyo is coming back, which gives him the willpower to successfully fend off Zeon's two Dragons while carrying Zatch on his back, without a single offensive spell. Then he sacrifices his book to save his partner, and tearfully apologizes for failing everyone- except Kiyo returns earlier than he expected to tell Momon that he did a great job just before he disappears. Then Kiyo and Zatch deliver the most epic curbstomp battle in the series to Rodeux, having already mastered multiple new spells that he's never used before. Then there's the whole battle between Zatch and Zeon, with the debut of the true Bao, everyone helping Zatch overcome its evil spirit, Cherish overcoming her fears and defying Zeon, and finally after Zeon's defeat and redemption- his giving Zatch his electrical powers to boost Bao even further and destroy Faudo in one shot before it destroys Japan.
more specifically when he used his own body to shield Li Yen, Tio, and Megumi from Unko Tin Tin's self destruct.
Tio shielding everyone from Clear Note's attack
In one of the episodes of Zatch Bell, when Zatch was hanging on trying to save Naomi from a big lion, all hope seems lost. Naomi, who was scared at first, looked up at Zatch. When the lion was about to crush her, she jumped very high and started climbing like there was no tomorrow to save Zatch and herself!
Two words (and a warning that happens at the last arc): BragoPunch.
Chapters 250-252 are a collective one for Monmon. When he learns that Kiyomaro is alive and returning to the battlefield, the little monkey finds his courage and devotes himself to protecting Gash no matter what. He fights without a single spell that can inflict injury, and runs rings around both Jedun and Rodeux, using his spells to their fullest effect(and learning two more mid battle), and his ability to sense magic to easily dodge both of them. He nearly wins the fight too, only failing when Elle receives an injury that forces him to save her first at the cost of his book. He does however buy time for Kiyomaro to return, which was his original intention.
A small, overlooked one for filler character Shion Hibiki. In the fifth and final episode featuring her (by which her spellbook has already been burned and her mamodo partner has left her) she's filled with a new resolve, snaps out of her Heroic BSOD, goes all Action Girl and throws herself at the arc's main villain in order to knock the MacGuffin out of his hand. This action ends up breaking said MacGuffin and allowing Zatch to defeat the bad guys. And seeing as said bad guys had screwedwithShion this whole story, it's definitely a case of Laser-Guided Karma.