The Elves and Pirahni eventually teaming up against Darkhell;
Jadina turning into Jadilyna in Book 6 and threatening Ceyderom into helping her to use the Temporhell.
Shun-Day threatening the village's mayor into submission in Book 7. While it was probably more intended to be a hint toher true nature, this was still pretty cool in some way.
Darkhell's and Elysio's Fusion Dance against Anathos. The resulting "Super-Sorcerer" was so powerful he could actually negate the Jovenia Incident's effect on himself and turn back to adulthood. And even though Anathos won, their fight was pretty awesome.
The fights between the Legendaries and the Hellions. Initially, each Legendary only ends up being beat up by his/her Hellion counterpart, while Anathos has Dark Jadina bringing Jadina to him and shows her through a crystal how her friends are bitting the dust. Then Razzia, Gryf, Shimy and Jadina reveal their power upgrades and single-handed defeat their respective opponents.