These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anti-Climax Boss: For a creature that's supposed to bring about the apocalypse and features a moveset full of One-Hit Kill attacks, baby Levanthan is surprisingly easy to deal with thanks to his extremely long tells and ease in which he's knocked down (one of said One-Hit Kill attacks even ends with him tripping and falling over all by himself.)
Awesome Music: Tunes like "Rise to Action and Invasion" or "Valkyrie March" really hit home.
And hitting their swords back at them is easier said than done. It's easiest when they spawn swords right in front of them, but rarely will they actually do that, and there's usually so much stuff flying at you from all directions that even when it does happen, you probably won't be able to take advantage of it in time. And make sure not to hit the wizard at the same time you hit the sword, or he'll just teleport away anyway (if he doesn't take the sword in the face and then just instantly teleport just to be an ass.)
Any enemy with a stomping attack is usually this. Bonus points to the guards in Titania (a.k.a. the "Butt Brigade") for appearing alongside the aforementioned Beldor boss fight.
The Butt Brigade's stomping attack is also the quickest and hardest to dodge of them all.
Designated Villain: Odette. On the one hand, she's a massive bitch. On the other hand, all she really does in-game is try to enforce death's finality. She has even less ambition than Elfaria and, aside from Oswald's case, wouldn't come into conflict with the heroes at all had they simply stayed in the land of the living.
Draco in Leather Pants: Ingway. Here we have a guy who allowed his sister to be abused by his grandfather while he himself hid from punishment, betrayed his country and caused the disaster that destroyed it, permanently curses the man his sister loves in order to keep them apart, and generally manipulates and dicks around with almost everyone while blaming his mother (who was actually blameless) and his grandfather for all of his misfortunes. A good bit of what causes Armageddon can be traced back to him; even his more selfless actions, such as inflicting the pooka's curse on Beldor to save Mercedes and transforming himself into the Beast of Darkova to try to stop the Cauldron, only serve to make things worse. It's a good thing he's pretty.
Ending Fatigue: Actually more like Game Fatigue. How many times are you willing to go through the same exact zones? How many times can you stomach fighting the same exact boss who has the same exact tactics? How many times are you willing to have the same story retold to you but through a different perspective? How many times can you put up with building a character up to a complete god but are reduced to less than a peon and have to start over because you're on the next chapter?
Freud Was Right: Without his huge flail Odin's powerless... wait a minute...
When Odette lifts up her skirt during her boss fight, the player gets assaulted with lots of spidery legs and a giant gaping maw. Yeah...
Game Breaker: The Adversity Ring is far and away the most powerful accessory in the game, capable of giving your characters a MASSIVE damage boost when their HP is low. Especialy considering it STACKS with the Overload spell.
Mercedes + Unlimited Power potion = Nonstop magical machine gun of pain.
The fireball and ice crystal enemies tend to pop up at the worst possible times during certain boss battles.
The animal ghosts in the underworld. Be sure to use Blizzard Potions!
How about those flying eyeball enemies with the stun ray attacks? A tough boss fight is turned BRUTAL by those guys.
The kings (er... queens) of this are the damn fairies, whenever you're fighting against Ringford - flying, almost impossible to hit, and they fully heal whatever you're fighting. When you fight a boss that has fairies supporting it, you want to throw something straight through your TV.
Jerkass Woobie: Ingway. He's selfish and manipulative and instigates quite a bit of trouble, but not entirely without reason: he caused the destruction of Valentine to save his father, Demon Lord Odin, who promptly rejected him. He was forced to choose between betraying his mother and claiming he didn't love her or most likely being executed along with her, and believed for years afterward that his mother had cursed him and died hating him. Despite the awful crap he pulls throughout the game, he demonstrates that he carries a tremendous amount of guilt over his past actions, and genuinely cares for both Velvet and Mercedes. Ultimately he tries to sacrifice himself to stop Armageddon by becoming the Beast of Darkova to stop the Cauldron, only for the effort to go horribly wrong in almost every way.
Nightmare Fuel: King Gallon is a three headed skeletal beast. And fighting the guy for five hoursdoes not make it easier. The Netherworld also plays on lingering childhood fears of the dark, and is a generally creepy and depressing place. It doesn't help that during the fight with Gallon, you can see Titania literally crumble and sink into the ocean piece by piece in the background.
The curse he's afflicted with may even be one of the creepiest examples of Evil Feels Good ever. He's perfectly aware that everything he's doing is utterly horrible, but razing his own kingdom, slaughtering his people, committing filicide and even eating his own grandson alive in one of the bad endings just feels so good.
To get 100% Completion, the player needs to unlock every cutscene. This means intentionally choosing the wrong order in the Final Book and watching the Bad Ending at least three times. Myris's sobbing while the world collapses and the oceans are rising to kill her while asking why someone would choose this horrible fate doesn't hits especially hard.
Sending Mercedes to fight the beast of Darkova in the Armageddon book is guaranteed to make you feel like an asshole. Her fate in the good ending isn't much better, either.
Tear Jerker: The good endings are painfully bittersweet. The bad ones are even worse and usually without the sweet.
That One Attack: Onyx's inferno charge can cross the entire arena (twice) and leaves a field of damaging flames in its wake; in addition, Oswald can't jump high enough to clear it. Heaven help you if Onyx decides to spam it.
He also lunges at you if you try to jump to stop you. Fortunately, he stops charging after this, and this can be exploited by running in the other direction and doing short hops until his lunge is triggered, and then ducking as he flies over your head.
Brigan's grab attack isn't a One-Hit Kill, but it may as well be, dealing insane amounts of damage and dizzying you afterwards. Unless you've been chowing down with Gwendolyn to raise her max HP, if he grabs you, it's over.
Belial's inhale attack, where he attempts to suck the player in and eat them For Massive Damage. This usually comes after he's filled the arena with junk, so there's lots of things to hit the player as they're trying to run away and knock them backwards (and of course Mooks aren't affected, so they're free to further help knock them into Belial's maw.) On the plus side, if the player managed to get far enough to get behind him, they can wail on him with impunity.
That One Boss: Another proud Atlus tradition.* Technically, Atlus was only the publisher, but as hard as these bosses are, you'd swear their influence snuck in the development anyway.
KING GALLON. His heads constantly try to devour you, and the only way to defeat him is to strike his heart. Which will only appear after you've sufficiently damaged the heads. And when it does, you've opened a whole new can of worms.
Let's give a special mention to Velvet's version of this fight, shall we? Because not only is she tasked with taking down an enormous dragon and wizard at the same time, the wizard has a healing spell. An if he uses it, he can restore said dragon, which has a buttload of HP, from near-death to full HP.
As well as the sheer amount of lag when fighting Odette, Queen of the Dead, forcing much of the battle to proceed at an insufferable crawl, and that's not including when she knocks you across the room to start healing herself.
Odette is a mercy compared to fighting Belial as Cornelius, where the game just turns right into a Bullet Hell game. Along with Belial's own massive amounts of projectiles and melee attacks that draw the player in and do massive amounts of damage, he's also accompanied by a respawning elite wizard enemy to add even more projectiles. At this point, the lag is practically a mercy as well, just because it makes it slightly easier to dodge the ridiculous amount of things falling on you and flying at you.
That One Level: Titania Capital, for every characters. This level tends to come with some of the worst Goddamned Bats and Demonic Spiders in the game. It's pretty much quasi-impossible to beat without very good preparation.
It could explain in gameplay terms why Gwendolyn, the first playable character, never has to go there.
Adding to that, every single boss fight in Titania is That One Boss. The Three Wise Men are fought there, along with the aforementioned fight with Belial, and the fight with King Gallon.
It's far worse with Cornelius. While the game expects you to fight the purple wizards and slimes in a specific way, with the other characters, there's at least a special attack you can use to bypass that and just cheese them to death (Mercedes' charge attack, Oswald's shadow form, Velvet's homing attack.) Cornelius, however, doesn't have one, meaning you have to fight them in the intended manner.
Wangst: Subverted. Oswald's Death by Despair when Gwendolyn hands the ring he gave her over to Odin seems kind of wangsty at first, until you finally get to play him and learn about the kind of stuff he had to go through. Namely, learning that his adoptive father sold his soul to Odette to give him the ability to wield the Belderever, and only cared about him as a tool. For all he knows, the girl that he loved, and really the only other person he really cares about, just used him as well.
The Woobie: Belial. One of the characters with the most tragic backstories in the entire game. Dragons already have a hard time in this game, being universally feared and hunted by pretty much every sentient race, but Belial spends the entire game under submission by the Wise Men and forced to battle the heroes against his will. He even thanks Cornelius for mortally wounding him after their fight, since it allows him to finally be free from the Wise Men's control.
Woolseyism: Averted, oddly enough for Atlus. The game was a strange case where the seemingly Shakespearean dialogue was almost entirely the result of direct translations rather than adjustments. In fact straying from the original script was what made for awkward, toned down parts such as Mercedes's Rousing Speech. This might make it an Inversion.