Affably Evil: Advocat, whose goal is ultimately to get people to sign away their souls in a Deal with the Devil, but who still genuinely seems to enjoy his friendship with Gammel Dore and the others at the school. He's actually on your side in every iteration, but he admits himself that it's purely because he can't get anyone's soul if they die first.
Only in the localization. Gammel Dore just had a name change to appeal to the Harry Potter fans in America. Advocat may be a snarker, but he and Snape are on two completely different levels.
The Computer Is a Cheating Bastard: They aren't restricted by silly things such as resource levels or build times—in fact, you can even catch the computer in the act of teleporting in more Mooks for it to use.
Crippling Over Specialisation: Affects the Skullmage, who's role as chief killer of ethereal units is easily taken by other more versatile creatures.
Unless Hell is run by individuals like Advocat; in that case, they'll have a field day at every poor schmuck demon that actually falls for the trick.
Speaking of Advocat; his deal with Gammel is that he will "teach and assist" at the school for no pay; but should Gammel ask for a Deal with the Devil for his soul; he'll get it. Gammel never succumbed, even in timelines where he's about to be possessed by Grimlet.
Did You Just Scam Cthulhu?: Lillet successfully pulls this off in the end with Grimlet, making contract with him where her request is for him to embrace god, which devils can't do, and since neither of them can change the contract (Lillet made it a blank check offer), he gets sent back to hell for breaking it.
Getting Crap Past the Radar: When Lillet first meets Advocat he starts harassing Amoretta, all the while making quite a few sexual remarks that you wouldn't believe made it into an E10+ game. It gets worse when you notice that before the comments start, Amoretta calls Advocat her uncle. There's also another time where Lujei, when speaking to the skull of her dead lover, tells the skull to 'tell me again what you whispered to me in bed.' Um.
The Gods Must Be Lazy: There's no other explanation as to why devils can walk around as they please and have to leave humans to stop them.
"Groundhog Day" Loop: Lillet seems to be cursed into repeating the same five days over and over again until she gets it right.
Hide Your Lesbians: Technically, Lillet and Amoretta don't do anything like kissing; but they do live together in the end. Snuggling in bed together is compared to one being "like a baby." Declarations of love can still be argued to be "sisterly." They never cross that ever so fine line of plausible deniability on the part of the writers.
It's Up to You: Of course it is. You're the protagonist! (Hilariously, in subsequent loops, Lillet will blow off helping certain people because she's got bigger priorities.)
Justified Tutorial: Lillet's a new student, and it's her first day of school. Of course, on repeat sessions everyone expresses considerable surprise at how much Lillet already knows.
Karmic Death: All the bad guys, thanks to some particular cleverness on the part of Lillet.
Knight of Cerebus: Both Calvaros and Grimmlet. They show up, the cycle is going to get much more serious.
Lightning Bruiser: Chimeras are massively destructive (and can actually out damage dragons since they have a higher attack rate) and move alarmingly fast, and ignore obstacles, but their HP gradually decreases. Also, while the game lists them as having close to the same HP as dragon, their defense is nowhere nearly as high so they die much more easily. Finally as alchemical creatures, chimeras get curb-stomped by dragons in a clash of the titans.
Logic Bomb: In a non AI example this is how Lillet wins against the Bigger Bad. Turns out that wishing for the Devil you just sold your soul to to "Embrace God" is a great way to get rid of them and get your soul back.
Meaningful Name: You cannot help but suspect Margarita Surprise from the beginning.
It's less obvious in the Japanese version, where her last name is 'Frozen'.
Mighty Glacier: Dragons. By far the slowest units in the game- but by FAR the strongest (most units die in one or two attacks, unless they're astrals, but using skull mage using astalize on the dragon can solve this) and by far the most durable. It's attacks. They. It takes an ARMY to bring down even one. Or a well placed sleep spell from a Grimalkin.
Necromancer: Opalneria. Also Lujei when she was alive. Also, Hiram is Opalneria's student which makes him an apprentice necromancer.
Spanner in the Works: Nobody, not even the highest ranked Demon or the Archmage, counted on one humble transfer student blowing all of their plans to hell.
Crossover Example: Lujei Piche ends up sending the demon Sulfur, villain of Phantom Brave back to his world despite the sacrifices involved in banishing him, as revealed in her Cameo in Soul Nomad & the World Eaters.
Stable Time Loop: Lillet learns after all her efforts that she will be trapped in the room with the stone like the other, older Lillet unless the stone is destroyed. She also mentions that when she arrived the Lillet she met was an old woman so it's pretty obvious that it's an unbroken cycle.
Stealth Pun: The sorcery teacher is a devil. His name is Mr. Advocat. And he's constantly flip-flopping his allegiance based on his mood. Put the pieces together. He's a devil's advocate.
Summon Bigger Fish: The only way to defeat Calvaros is to summon the arch-devil Grimlet, who takes his soul to hell. Of course, then you have to deal with Grimlet which is where things get difficult.
Wild Card: Advocat. He is a devil who could exploit the situation to his benefit if he wanted to, but he knows he has very little to gain personally from the conflict and is content with simply observing the events. In the ending however, he's so impressed with Lillet's outsmarting of Grimlet that he personally backs her deal with him.
Wishplosion. An incredibly epic one. Lillet summons a massively powerful devil, which fulfils the Big Bad's contract with him, which gets rid of said Big Bad - and then Lillet drops a Logic Bomb on the demon, making a contract and then wishing for it to embrace God. The demon refuses and therefore suffers the consequences.
Zerg Rush - On the first couple of days, you can just mass-manufacture your unit of choice and march them to the objective. Not so much on the later days, where your opponents tend to have a mix of unit types. Phantom swarms, for instance, go from awesome (when the AI tends to have little to counter them) to short-lived (when the AI gains Homunculi and alchemy area-of-effect attacks) to awesome again (when you figure out the subtle nuances of using other units to take out said Homunculi and area-of-effect defenses before sending in the crazy sword-swinging avenging ghosts soldiers).