Anticlimax Boss: Kagutsuchi. If you Buff and Debuff to maximum, he's mostly a joke, especially compared to the rather brutal battle that precedes him.
Sakahagi. Sure, you fight him right after taking down Girimekhala (the first boss to reflect physical attacks) and given no chance to heal, but he only knows second-tier elemental spells and can be easily taken out in one turn by this point in the game.
He does pose a mild threat in that he can easily finish you off if you're weak enough from the preceding boss fight. Especially if there's any elemental weaknesses in your party. But if you survive his first attack, you're more than likely going to be fine.
Breather Boss: Thor the second time. Sure, he'll start using Dragon Eye when he hits half health, but really, you should be able to drop him from his full 8000 to 5000 fast, and then finish him in one turn.
Mara, even though he heals himself to full health on every turn. His strongest attack is Hades Blast, which isn't that bad even though it deals a boatload of damage. You're also forced to fight him during a full Kagutsuchi, thus enabling Bright Might. Bring a few demons that have it, and he'll be dead before he gets a turn.
Taunt + Tetrakarn = Dead Mara.
To a much greater extent, Mithra is often considered by players to be the easiest boss of the game( not particularly saying much). The majority of his turns are wasted on Expel or Death spells, which are inaccurate and easily blocked by that point, meaning that even after using Dragon Eye, he'll end up not doing much. He is also quite frail for such a late-game boss, as he can be killed within less than 4 turns with little trouble. The only thing that can (not necessarily) mess you up in this battle is that you can't be immune to Death AND Expel at the same time at this point of the game (though your demons certainly can).
Black Rider is one of the easier Fiends due to not having any way to deal with buffs and debuffs and having no other gimmick like Daisoujou and Trumpeter other than being a Flunky Boss.
Darkness-Induced Audience Apathy: Everyone's a dickmunch in Nocturne. God, your former friends, the villains, the 'nice' reporter who decides he's going to be evil more-or-less out of nowhere, EVERYONE. Suddenly the True Demon Ending's not lookin' so bad, is it?
Draco in Leather Pants: Lucifer and the "True Demon" Demi-Fiend get this a lot. The fact that their agenda involves killing off the last bits of creation that serves the Great Will, save Lucifer and his "army", seems to be glossed over at best.
Goddamned Boss: If you come prepared and don't have Focus, Pierce, and Freikugel, Noah. Aurora causes the fight to drag on longer than it has to. If you're unprepared or don't know the pattern of his weaknesses, he delves into That One Boss territory.
Hilarious in Hindsight: Monster Hunter 4 Ultimate also features Dante from the Devil May Cry series.note Though it's just Palico armor themed after him as part of a monthly DLC campaign.
Les Yay: Fun fact, having a female demon use Seduce (high chance of recruiting male demons when used by a female demon) or wooing (high chance of recruiting female demons when used by a male demon) can work on other female demons, while a male demon using either on another male disgusts the target and results in a guaranteed fail.
It doesn't work so much as it doesn't not work. A female demon's chances of recruiting another female demon using Seduce are exactly the same as if trying to negotiate normally. One would imagine that female demons are a bit more comfortable about being hit on by someone of the same sex than male demons.
If Matador challenges to a duel, get your buffs ready because he is dead serious.
The Demi-fiend himself. His actions on the True Demon route and in Digital Devil Saga has amazed enough of the fanbase that, whenever questions of who the strongest SMT protagonist is come up, his only competition is Aleph.
Mot's Drama, which refers to whenever Mot spams Beast Eye and pairs it with enough Makakajas and Megidolaons to wipe you out in one turn.
Magnificent Bastard: Lucifer. He grants you the power of the Magatama as part of a combat test trial. It really doesn't matter what you do with it - literally anything you do with itends with a net gain for him. You go and make sure any of the three main Reasons wins, you prove the product is workable. You leave the Vortex World as-is, you spit on the Great Will's face. You restore the world as was, you spit on the Great Will's face and he gets a warrior that in due time will fight again against Him. You accept the offer to reach him in the Labyrinth's lowest level, he gets a worthy successor and new general...
Narm: Listen to Daisoujou when he uses a physical attack. White Rider using God's Bow is also pretty Narmy.
Whenever Trumpeter speaks.
Every time Black Rider uses a spell. Heeeeee!
Whenever Shiva does anything. Idle animation: dancing and jerking around like a mannequin on crack. Launching a spell: twitters like an angry bird. Doing a physical attack: "WHEEEEEEUUUU!"
He's less ridiculous than Shiva, but Vishnu's weird moan whenever he casts a spell is pretty darn Narmy itself.
Scrappy Mechanic: White Rider's spawn point is in front of every major Terminal after reaching Asakusa and completing the Second Kalpa. Pray that you'll be able to escape or beat him in one go...
Beast Eye/Dragon Eye, or any other move that grants the opponent free press turns. Mostly because it's one of the few things in the game that is blatant that The Computer Is a Cheating Bastard, with them using a move that is unavailable to the player, that costs only 1 MP, and sometimes even abusing it to outright win. And worse, depending on the enemy and the moves they use, it can be impossible to counter with demons that can nullify attacks.
While exacerbated by starting the game on Hard Mode, this page has a comprehensive list.
Beast Eye and Dragon Eye. After consuming the turn, the enemy gains two or four half Press Turns, respectively. This leads to the enemies (generally bosses, too) proceeding to attack you up to five times in a row. Daisoujou and Hell Biker also have Startle and Hell Throttle, respectively, that are the same but give three Turns.
Matador's Red Capote will automatically max out his hit/evasion rate and is one of the main reasons he's the game's Wake-Up Call Boss.
Trumpeter's Holy Melody and Evil Melody. The former fully heals whoever has the lowest percentage of HP, while the latter would kill him. While when he uses it is predictable, it's also unavoidable. In addition, the Trumpeter can use Holy Melody on himself, but can't use Evil Melody. Interestingly enough, the same applies when Trumpeter learns it in your party...and Evil Melody only costs 1 MP...
Matador, who gets the honor of being pictured in the Atlus That One Boss page. With the right strategy, he's not too difficult, but he's the first boss in the game that you won't be able to beat without fusing a party specifically to counter him. And even if you do he's still a pain in the ass, replenishing his Evasion faster than you can drain it.
Mot. He can use Beast Eye, but plenty of other bosses can, too. Unlike them, however, Mot isn't designed to do anything specific with Beast Eye - it's just a regular move to him, unlike most other times you run into it or Dragon Eye. Mot can also combine spamming Beast Eye with Makakaja and Megidolaon, meaning he can wipe out your team without you getting a single move.
That One Level: That fucking cursed hallway in the Second Kalpa. Your HP gets cut in half every 5 or so steps, there are level 60+ demons that always seem to get through Repel. To get out you have to go back through the hallway, though thankfully there are several shortcuts so you don't have to go the whole circuit.
The lower levels of the Third Kalpa definitely count, too. You have to run around and flip switches while being shot at by Dante. Get caught by him, and he knocks your party around and sends you back to the beginning. In the second part, not only are the switches in mazes of invisible walls, Dante resets them after he catches you. The top level's not easy, either; it's a gigantic maze filled with one-way doors, and some of them teleport you if a certain stat isn't high enough. The only way to be sure that you won't get thrown all over the map is to have all your stats at 25 or higher, which would require absurd amounts of grinding at this point.
That One Sidequest: The Labyrinth of Amala in its entirety. The mandatory parts put you through the wringer constantly and remind you it's particularly hard to assemble a powerful team capable of resisting continuous assaults (many back attacks, particularly) at the beginning. The optional Grave Run is also particularly tough, even with the endgame stats.
Puzzle Boy, a young Manikin that makes you play a minigame of 20 increasingly difficult puzzles if you want to obtain all the Magatama in the game. He's practically required as the magatama he gives you teaches the Demi-fiend all the second tier healing abilities.
What an Idiot: Ongyo-ki. As long as you keep hitting the real one, he'll do very little besides buff himself. He even does this after the effects are maxed out.