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.
Demonic Spiders: Beefy enemies fall under this. They don't flinch from normal hits, they do tons of damage, they throw you everywhere, and they almost always appear with normal enemies. Also, Fire Demons. Nigh-immune to fire damage, lots of HP, and they love to spam magic.
The Stove Faces of Full Moon do also, as they're the strongest physical-fighting enemies in the game, and they come out en masse. Taken to extremes with beefy Stove Faces...
All of the above doubly applies to most enemies in the game under Insane Difficulty, especially if you're going at it alone.
Fan Nickname: "Assfaces" or "assholes" for the Thieves and Icekimos due to their face obscuring hoods seemingly imploding.
"Funny Aneurysm" Moment: There's a good chance the Orange Princess was alive throughout the game, though her face was obscured through coincedence. One of the pictures in the Painter's room has a picture of an orange dress with a knife/sword in it. This makes it possible that Tricky killed her late into the game and stole her clothes. This makes the scene where she cries in the Industrial Castle even worse, as it is one of the last times you see her.
There's always the idea that, even though the Evil Wizard didn't kill her, he did intend to. Only makes it less dark by a small bit.
Game Breaker: The Industrialist and the Fencer. Their splash attack is what is referred to as the "boss killer".
Goddamned Bats: Literal bats, scorpions in the desert, and the Thieves all fit this trope. Stupid arrows.
Slimes on Insane. They take a very long time to kill, can be hard to hit, and hardly deal any damage, but can jump on you and prevent you from moving. Both the Insane and Normal variants can screw up the synchronization of an internet game when there's lag involved.
Good Bad Bug: The boomerang glitch lets you grind level way faster than with standard game play.
Removed with a patch, though still possible on unpatched versions.
If you stand in a certain spot when fighting Pipistrello he never stops attacking you, which means he never takes flight or summons minions, and his attack is slow and blockable which lets the player get in a lot of hits. Abusing this is generally accepted as the only way to beat him solo on Insane.
That One Boss: The Catfish. While it may not be hard to avoid its attacks, every other boss lets you bring your friends back if they go down. However, on Catfish, when your 3 friends go down 15 seconds in, it's one Level 10 player doing 1/4 damage versus a 600 HP behemoth with 1 5-second window for damage every minute. In other words, 10 minutes of hearing nothing but "Screw this game, man, let's play something else." It doesn't help that the boss is heavily armored unless it is momentarily stunned by the King's cannon, but the cannon has very precise and finicky aim.
To make matters worse, assuming you're playing multiplayer in such a way that you can't communicate quickly, if your friends DO survive they can end up screwing the fight up by drawing the catfish outside of the cannon's fire zone.
If you picked the Industrialist or Fencer, and shoved your points onto magic, that boss would have been dead before you said "oh, hell yea"
The Conehead Groom can also apply if you're the bare minimum level. He's fast, hits harder, is almost impossible to juggle, and can rain a volley of bombs down on you with his cannons.
The big troll mid-boss before the Abandoned Mill on Insane difficulty is significantly harder than anything you run into for the next several stages, including the Catfish and Pipistrello. This is mostly due to the huge numbers of minions he spawns and the amount of damage they deal on Insane, while the boss himself has a lot of health and, at low health, runs away every time he takes damage. It becomes an attempt to stay off the ground as much as possible by 'crowd-surfing' with air combos until he dies.