Abnormal Ammo: Cadderly uses a hand crossbow that fires exploding darts.
Anti-Villain: Dorigen. Aballister has shades of this, as well. Also Rufo
Badass in Distress: The entire plot of the fifth book, where resident ass-kicker Danica is incapacitated and captured early on.
Bash Brothers: The trope might as well be called The Bouldershoulder Brothers.
Battle in the Center of the Mind: Happens a few times. Cadderly engages in a battle of wills against Druzil, and later an epic Professor-X style knock-down drag-out with Ghost.
Beware the Silly Ones: Ivan and Pikel are the Abbott and Costello of the series. They are also the most dangerous fighters besides Danica. On the villains' side, Druzil is rather hard to take seriously, though he is behind everything bad that happens in the series.
The Chick: Danica, but mostly because she's so badly injured.
From Bad to Worse: In the fourth book, the heroes finally defeat evil sorcerer Aballister and destroy Castle Trinity and the evil forces that have been behind the entire series. Time to head back home to the good 'ole Edificant Library and Oh, Crap Rufo has consumed the Chaos Curse, become a vampire, killed Dorigen, captured Danica, and turned the entire staff into murderous monsters. Druzil has also defiled the Library so divine magic is almost impossible to cast inside of it, where Rufo has set up his den of vampires.
Mark of Shame: Kierkan Rufo gets one burned into his forehead by Cadderly after betraying the group and indirectly killing Headmaster Avery.
Our Dwarves Are All the Same: Played straight to the point of parody with Ivan Bouldershoulder, and completely averted with his brother Pikel.
Religion of Evil: Aballister and his minions worship Talona, the evil goddess of poison and disease.
Screw You, Elves!!: Ivan's entire dialogue whenever ANY elf is around, though it's mostly bluster.
Shapeshifter Mode Lock: The druids in the first book suffer this under the effects of the Chaos Curse, getting stuck in the form of their spirit animal. Newander is actually upset that he's still a human.