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.
Breather Level: Chapter 7 is definitely this, since it's very short and their isn't even a single enemy throughout the whole thing.
Goblin Bombers chuck bombs that are hard to see and harder to avoid, and will blow you sky high if you're near them. To make matters worse, they always accompany other Goblins, making it harder to focus on killing them.
Orc Warriors and Orc Captains have ridiculous amounts of health, are very fast, and their shields can block most of your attacks unless you break it with a solid Projectile or hit them while they're charging.
Goblin Captains also have shields, but they also have Frost Cleavers, which Freeze you with every hit. Yep, every time you get hit, you get Frozen solid. Again. And again. And again.
Elementals are fast and tough, nothing special. But what sets them apart is the fact that you have to hit them with their Opposites. This means that if you're fighting more than one at once (which you will be), you can only hurt one at a time (and may even heal the other one). Also, Elementals are awakened by you being near them or by being hit with attacks. If the Elementals you're fighting don't chase you into all the other ones, your stray shots will wake them up just fine.
The infamous Snow Trolls are just plain absurd. They pop out of nowhere at the speed of sound, will relentlessly chase you around what little room there is, and can one-hit-kill you in the blink of an eye. They also have unfair amounts of health AND regeneration.
Malignant Watchers are also living nightmares. While they aren't nearly as fast as Snow Trolls and can't take as much damage, they more than make up for it with massively damaging ranged attacks. Their lightning bolts can drain Personal Shields very quickly, and if you decide to drop your Shield and go with Lightning Resistance, it'll use its main eye to insta-freeze you, then insta-gib you.
All enemy Spellcasters are tough, but Dwarf Priests just take it too far. Between having loads of health, Arcane Resitance, Teleportation, and a love of massively damaging Earth Projectiles, their only weakness is that they're slow.
Shoggoths seem to be The Stars Are Left's answer to the Snow Trolls. Remember how At The Mountains Of Madness originally compared one to a subway train? Well, they're about that fast and durable, and even more likely to kill you if they so much as touch you.
From the same expansion, Endermen. They attack in groups and will relentlessly pursue you at high speeds, and they can Teleport. They also have very powerful attacks.
Elder Things are a special case. They don't move and have only one attack, but that attack is Confusion, which inverts your controls. There are several that are perfectly positioned to be in your way, and they Confuse you the instant they take damage. This means you're REQUIRED to kill them in one hit or else have your controls inverted.
Goddamned Bats: Goblin Archers. The arrows they shoot don't deal too much damage, but they do have this tendency to knock you down if you're using a shield. And by the time you get up, they have another one notched, ready to knock you flat on your ass again. And this is only with one; against multiple Archers you may as well just lean back and let your wizard's Shield drain. Gives a new meaning to Annoying Arrows.
Beastman Raiders, mainly as a result of their ability to hop around. They just will. Not. Hold. Still.
Daemonlings might be the worst Goddamned Bats in the game. They always appear in large groups and are very fast, so before you know it you'll be surrounded. They attack really fast and they're attacks knock you around and cancel any Spells you had lined up, and they're so fast that by the time you've recovered they'll be right next to you to hit you again. Water Sprays and AOE Spells are the only things with a chance of killing them before they start punting you around like a volleyball.
EVERY SINGLE ENEMY becomes this in Vietnam, due to everyone having guns.
Cultists from The Stars Are Left. Take everything that made Rangers and Archers annoying and give them more health and more damage, and a tendency to hide behind other already tough enemies.
Goddamned Boss: Jormungandr, because of how awkward it his to attack him. Someone has to be right next to him, and you have to hit him in the head (which is a small target and can be tricky to hit due to the weird angles and perspectives) in the moment before he close range attacks. If you miss, you have to wait a while before you can try again. What's more, sometimes he'll use the long range attack even though there's someone standing right next to him. An easier way to fight him is to summon rock pillars- if he bumps into them, he'll take lots of damage and retreat into the ground. Rinse and repeat.
The second fight with Grimnir, for one simple reason: when you fully deplete his health bar, you have to cast Corporealize (which is pretty complicated) very fast, or else Assatur will heal him back to full and make you fight him all over again before you get another chance.
Idiot Plot: The entire second half of the plot. Why would Vlad not explain the situation to his past self? Or, why didn't he tell the heroes where Fafnir lived himself? If the Wizards can't talk, why didn't he talk to Death and Fafnir? In fact, why send them back in time in the first place? The answer to all those questions is you shouldn't care, because the whole thing is played for laughs.
Most Annoying Sound: Every time the Exposition Fairy speaks. Which is a lot. While it's supposed to be a parody of expository companions in games (Navi being the most obvious example), some people feel it worked a little TOO well and are compelled to kill themselves for the sole purpose of shutting her up.
That One Boss: Sweet merciful lord, GRIMNIR! Even the developers had trouble beating him. As if fighting an enemy who could cast devastating Magicks, and Shield himself wasn't enough, he also has an indestructible Ethereal duplicate who can shoot Projectiles from all angles, a tiny arena surrounded by a bottomless pit, AND the Mind Duel.
The Aristocrats are also pretty damn tough. Mainly it's the length of the battle, combined with having to fight multiple enemies at once. Things really get hard in the final wave, however. You have to kill the black-robed Aristocrat, who can use all the abilities of the other Aristocrats, and constantly resurrects the others to ensure that there's always five onscreen at a time. If not being able to kill his minions was annoying, the fact that he hides behind Shields is just the icing on the cake.
The final boss of The Stars Are Left was created with the intention of being the hardest boss in all of Magicka. Does he succeed? Let's put it this way: He has no less than THREE attacks that can instantly kill you, AND an instant-death pit around his tiny arena. And minions. And a load of health. And just when you thought it couldn't get any worse... OH SHIT DAGON.
That One Level: Chapter 6. There are bottomless pits everywhere, and the whole level is crawling with all the various types of Daemons, who have such wonderful abilities as high speed, high damage, superhuman accuracy, the ability the Phase into the Ethereal Realm, where you can't hurt them, and splitting into more enemies when they die.
Chapter 8 is another contender, mainly due to its length and the over-abundance of Goddamned Bats.
The crown, however, has to go to Chapter 11. It's the longest level in the game, and has enemies practically every few feet, and there is not a single enemy in the level that could possibly be considered weak or easy. Snow Trolls, Dwarves, and Malignant Watchers? The game's most infamous Demonic Spiders? ALL FROM THIS CHAPTER.
The Caverns Challenge map is arguably the hardest one available. The first basic thing that makes this level hard is that it's set on a very small platform that makes manoeuvring very hard. Since it's set underground you can't use weather Magicks, Lightning Bolt, Summon Phoenix, or Napalm, which means that collecting those Magicks from treasure chests becomes useless. You also have to face waves upon waves of Dwarves and Malignant Watchers. And to top it all off, the enemies will sometimes use the elevator in the center of the stage to attack, which means that if you're not careful you'll find yourself being attacked from all directions at once, including from below.
Chapter 3 of The Stars Are Left: R'Lyeh. After the foes you've faced in Magicka, what could possibly be a great threat to you now? One word: PUZZLES.
Unfortunate Implications: Magicka: Vietnam. The enemies are pretty much the same as in the normal game... meaning, you're running around Vietnam, mowing down hordes of short, ugly, stupid, yellow-skinned Goblins. Um...
On the other hand, real Vietnamese couldn't manhandle heavy machine guns like Trolls.