Pretty much any time the player is at Hu Lao Gate on the Coalition side and forgets that pursuing Lu Bu is optional. (Dynasty Warriors 6 lampshades this briefly when Yuan Shao, responsible for the infamous meme in DW 3, simply says that if no one can stand against Lu Bu, "Then forget about him!")
Oh, and as for 4, two words: Nanman Campaign. First off, you're up against a massive, highly motivated force. With elephants, which your forces have trouble with. Likewise, armor troops. Plus after a few seconds, your side constantly loses morale due to the harsh climate. And every time you beat Meng Huo outside of his headquarters, he returns more powerful than before. Plus the enemy gates continually regenerate, meaning that they have effectively unlimited forces. It's so bad, any strategy other than "go right to enemy HQ and take out the commander quickly" is practically suicide. An archtypical case of a company legitimately wanting to create a challenging level and Not. Knowing. When. To. Friggin'. Quit.
It gets worse: There's two different versions of this level - Wu's version actually nerfs Meng Huo (he won't respawn all those times), Shu's version is where you actually get to unlock Meng Huo and Zhu Rong. How do you unlock them? Zhu Rong is simply unlocked by defeating her in a duel (as Shu, after dueling Meng Huo). Meng Huo, on the other hand, requires you to defeat him all 7 times, one of which MUST be done by duel. The ONLY way you're pulling this off is to break the 1000-KO barrier to max out your force's morale while taking out enemy officers so that your commander (and the bodyguards you hopefully didn't forget to have wait at YOUR HQ) aren't taken out.
There's an easy way out for this one, however: Equip a horse, then make a mad dash to Meng Huo's base. He'll respawn there and if you beat him there, he'll be beaten for good, finishing the level. Of course, that means you miss out the bonus stuffs as listed above.
The Two Qiaos also qualifies. The situation: You're either Sun Ce or Zhou Yu, and you have to find both the other and the eponymous Qiaos, all of whom are scattered across the battlefield. You're up against lots and lots (and lots and lots and LOTS) of enemy soldiers who pop up in massive ambushes all over the place and have no troops or bodyguards of your own. And if anyone goes down, YOU FAIL THE LEVEL. (This stage, incidentally, is the only way to unlock the Qiaos.)
"Dynasty Warriors 6," Chang Ban, Wei Scenario. Trying to get to Liu Bei is hard enough IF you kill the peasants, but you have to avoid them if you want to complete one of the targets. You also have to kill Zhang Fei, who gets a power charge that makes him comparable to Lu Bu. And if you DO kill the peasants, and you kill more than one, KOEI's poster boy Zhao Yun comes chasing after you. If you don't kill them, he comes after you anyway. And this is all while having to deal with the time limit which extends to the moment Liu Bei gets to the boats, which is pretty damn quickly. Hell, if you kill all the chieftains as well, Liu Bei will stop and go after Cao Cao...and rip him apart in SECONDS.
You can send the innocents to their deaths provided the chieftains does not die, taking out Zhang Fei and Zhao Yun can be done by getting a friend with higher stats and nicer weapons...But indeed Chang Ban is just one nasty level for either side. Whereas the Wei has to complete their objectives before Liu Bei goes to the docks, the Shu has to deal with the fact that NO chieftains are allowed to die.
Dynasty Warriors 5 has Battle of Mt. Ding Jun on the Shu side. Not that the battle is hard by itself, but about ten minutes in, a painfully obvious but impossible to prevent sabotage occurs and Wei starts bearing down on both the commander Ma Su and the main camp, which are suddenly a long, roundabout way away from each other. Ma Su is a generic officer easily overwhelmed by the 3+ named officers bearing down on him; if he dies, you lose. The main camp is protected only by a handful of officers who are outmatched by the attack force; if both checkpoints inside the base are taken, you lose. The game has the courtesy to tell you what's going on, but unless you pick one issue, deal with it quickly and completely then hustle to the other, you'll lose before reinforcements arrive. This is one of the few maps where even advanced players can lose by mission failure if they drop the ball at all.
Dynasty Warriors 6: Empires has a whole bunch of Escort Mission levels, the worst of which are probably the "Deliver Supplies" missions. Two groups of fragile villagers on opposite sides of the field, who can't wait long enough for you to escort them one at a time, who insist on charging into every possible base (aka enemy respawn point), forcing you to run like hell back and forth to keep them from getting killed. If either group dies, you fail the level. And just because it wasn't annoying enough, the villagers are frequently attacked by tigers and wolves, which are like the Dynasty Warriors version of Goddamned Bats. RAAAAARGHHH!!!! The only good thing these missions have going for them is that they're mostly optional.
The peasant ones are the tough ones? Just get a horse (buy one and equip at start or press Select ingame) and you'll find them easy. Now the ones with Lady Cai Wenji are tough, moreso since she refuses to attack in self-defense (unlike the similar escort missions where you are escorting either a musou officer OR a custom character, who DO attack enemies).
Escorting those goddamn villagers was a nightmare (even with Red Hare).
Dynasty Warriors Gundam 3 had a final stage that was a complete massacre for most players. You start with only one other character on your side, who will probably not last more than five minutes. You have to capture multiple fields within a very limited amount of time, since the enemy missile bases (the areas that launch nukes at your side, which will completely turn one of your ally zones into an enemy zone when they touch down) will make short work on your HQ. (Which will take away your ability to respawn, even if you recapture it.) Meanwhile, you have to defend your zones from the constant attack from enemy Ace Pilots, who have outrageously high stats and ungodly health. The chance of you surviving a single hit is very small, but that doesn't matter, since the A.I. will usually propel you into a 4 to 12 hit combo. The enemy team has four of these Aces on its side, who work together quite nicely to corner you. Taking down one of these Aces takes a couple of minutes when they are together, and by the time you've started working down another one, the first Ace has already respawned. To complicate things even more, the enemy starts off with a 'Mobile Suit Factory', a field that slowly recovers that team's 'battle gauge', the meter that depletes upon an Ace's respawn. The A.I. is so powerful that whatever part of that gauge was lost from shooting down that single enemy is already recovered by the time you managed to defeat another, so you'll be harassed by these officers the ENTIRE MISSION. If you managed to overcome the complete unfairness, you're rewarded with a battle at their HQ against three heavily powered opponents. Two enemy Ace Pilots come out in the Musha and Musha Mk. II Gundams, extremely powerful mobile suits, and are led by the Knight Gundam, easily the most overpowered Gundam in the entire game. You must defeat all three of them to complete the mission. Is now a bad time to remind you that you cannot absolutely take a single hit? Knight Gundam seems to know this and will spam his SP attack every other minute.
Save Scumming is the only way you will beat this stage without breaking a controller.
Interestingly enough, there's something about this stage that isn't present in ANY other stage at all. Your ally mooks are actually useful, and will utterly destroy a single enemy Ace Pilot if they attempt to capture your zone by themselves. That's right, your generic grunts, who couldn't defend your zones for ANYTHING in the past 100 gameplay hours, are now suddenly strong enough that they can defend the zone AND take down the very same pilot that can kill you in one or two hits. A Crowning Moment of Awesome if you think about it long enough.
Hades felt like Filler and raised some serious Fridge Logic issues regarding Kratos' suicide.
This editor cracked up when, after dying one too many times to the terrain in Path of Hades, the game prompted him to switch to easy mode, with the note that "this will only affect combat." Yes, because that would certainly help me deal with niggling jumps and climb your blasted whirling bladed cylinder of pointy doom.
The extras disc for God of War II reveal that the Hades level was the only one that wasn't play tested, hence the difficulty.
Some people find every obstacle course and puzzle sequence in that game to be a Scrappy Level/sequence. They worked fine and all, but they were damn hard/annoying. Especially outrunning those spike walls, and dragging that cage up the mountain.
The worst was where you had to move a box along a floor in a certain time limit, or else spikes would come out of the floor and instantly kill you. I got through Hades with no problems (no, I have no idea how), and it took me a ton of tries to get past that damn room.
The above was lampshaded in the Collection edition with the achievement: "Kickboxer"
The first God of War also had one major Scrappy combat section: a confrontation with three Cyclopses early in the Athens Town Square map. On easier difficulties it's merely annoying, but on Hard and Very Hard, it becomes ridiculously frustrating and unfair, unlike the rest of the game which is hard but fair.
The final boss fight strips you of all the weapons you have spent the rest of the game learning and upgrading, and gives you a new one with a different set of moves. And the boss isn't a pushover, either.
On Titan Mode, every part of the game that's merely tedious turns into That One Level/Boss. One notable example is at the end of the first Pegasus segment, right after the prologue. The Dark Rider portion, specifically: he traps you in a draft and shoots bursts of dark magic at you that you have to dodge, the last of which is unavoidable and ends up railroading Kratos into the next section of the game. The problem is that now, each hit takes off over half of your life bar, so you have to avoid everything up to this point or you're dead. The only problem is figuring out how to dodge THAT DAMN PENULTIMATE DARK BURST! Even when you go on the opposite side of the draft from where it'd logically hit you, you still get damaged by it!
By itself, one of the late-game sections is tedious. You're basically grabbing the chains in a pillar and pulling them to make the elevator move down, but a ways down, the ceilings sprouts spikes and starts coming down, so it turns into a race to get to the bottom and open the door before you're impaled and crushed. Problem is, the skeletonsyou saw just lying on the ground since you entered start coming to life and attacking you. Not only do you have to keep them off of you while you move the elevator (which is easy enough with a cast of Cronos' Rage), but the skeletons will occasionally grab hold of the gears actually moving the elevator and stop them entirely, so you have to stop and kill them anyway. The whole thing is more tense than anything.
And of course, who could forget the gauntlet of enemies preceding Clotho's Chamber in the end game? Little-to-no health/magic recovery from beginning to end, and you have to face several rounds of almost every enemy you've ever seen in the game, from Satyrs to Cyclopes. Hope you mastered using The Golden Fleece, or you're not walking out of this with minimal damage.
The Cavern from God of War III. You're on a moving platform in the form of a giant box held up by a chain. The basic minions attacking aren't difficult, but then two minotaur appear, determined to break the chain, which is followed by another mook rush. Followed by two more with archers sniping at you. More mooks follow, and then SEVERAL more attack. And if you fail at any point, you have to start over.
The Trials of Archimedes from God of War: Ascension is another contender. Basically you must endure three waves, each with powerful enemies consisting of lightning sirens, gorgons, harpies and other powerful enemies. The real kicker is that there is little to no health powerups and if you die, you have to start from the first wave all over again. It becomes so bad that a patch is later released so that you regain health and magic for each waves cleared.
Mission 19 in Devil May Cry 3. There's this bit where the Demonic Spiders known as Abysses appear and you have to kill them off before the huge hourglass in the background runs down. If you don't, the Abysses reappear but your damage taken and items used stay as they were right before the hourglass had reset. Then it caps off with the horrible blob Arkham, who is most certainly That One Boss. The Boss Rush levels in 3 and 4 also fit to some extent.
Earlier in the game we have the 'Trial of the Warrior' which should translate to a fun amount of fighting against enemies. However...it becomes less fun when they can hit you through their own allies (which they aren't supposed to be able to do) and then the little...Demonic Spiders that move real quickly appear and make it virtually impossible to kill them. On top of the sand warriors and everything, it all accounts to one HUGE Scrappy Level.
Level 8 was a bit of a bum note as well. Dark, cramped areas, occasioanlly dodgy camera, and the floor can hurt you in places. Manageable on the easier levels, but when the game ramps it up, it just gets absurd. Plaing it through on Heaven or Hell and you'll need every bit of luck you can muster to get through without wanting to kill someone.
The Desert Lands levels in Gauntlet Dark Legacy. The enemies are so tough and numerous that you can lose hundreds, if not thousands, of your hard-earned Hit Points.
The Ice levels are all That One Level - they are all very long, the enemies spawn quickly and do NOT give good experience, there are traps that are hard to avoid which can be a real drain on your health, and getting all the necessary extras requires a lot of steps.
In Drakengard, we have Leonard's Regret, specifically the verse "Gleamings". Aside from the waves of those ever-annoying bulb-armor knights, the end of the stage sees you fighting a dozen heavy cavalry. They have an attack that causes them to charge at you and send you flying if it hits. Since there are so many of them, several are guaranteed to charge at once, and Mercy Invincibility doesn't kick in until you recover, so prepare to be ping-ponged between several of them. Oh, and they love doing this while you're busy attacking one cavalryman and they're offscreen. And there are two more waves after the first. Best to call out Leonard for the last one...
In truth, all of the pact-partner side chapters are like this. "Arioch's Madness", due to miserable flight controls, is worse yet.
Bayonetta. You've played through Normal mode, had a blast, punched Godinto the sun, and you've unlocked Hard Mode. Shouldn't be too bad, right? Unfortunately, in Hard Mode, Level 1 is that one level. Halfway through the level, you're on the wings of a plane, in Witch Time, and because it's the tutorial level, this is the part where you learn about torture attacks. You must execute 3 to pass this point. They don't redo the tutorial itelf, but you still gotta do the torture attacks... against Gracious and Glorious. To do a torture attack, you spend 8 units of magic. You get one unit for every few hits you land on an enemy, and lose 4 if you get hit. On normal, this wasn't so bad, because the enemies were pathetic. On hard, it's hell, because Gracious and/or Glorious just love to interrupt your combos, and are as aggressive as sharks in a feeding frenzy. The rest of the game isn't anywhere near this bad, and it's humiliating to get your ass handed to you sixteen dozen times on the tutorial level.
Act 3 of Diablo II. You have to slog through a jungle with switchbacks and dead ends populated by a) native pygmy men who either swarm and stab you or shoot you from afar with blowguns, which are led by shaman with the ability to revive the pygmys; b) enormous mosquitoes that drain your stamina and poison you; c) literal Demonic Spiders that are larger than you. This is on top of an act-long fetch quest which forces you to fully explore the jungle and the sewers of a nearby city looking for the unusually well-preserved remains (eye, brain, heart) of an old wizard.
Sengoku Basara: Samurai Heroes has Incident at Honno-ji. Apart from being a pain in the arse to even find the level is swarming with a wide variety of enemies, is large and time-consuming to navigate, and puts a strict time limit on capturing bases - and failing to do so allows That One Boss at the end of the level to resurrect another time, further putting your survival chances in the toilet. There is a trick to it: Nobunaga always activates the Mortal Coils in the same order. Take the southernmost base, then both northern ones, then use Hero Time to get to the south-western one. That base contains a slingshot to the north-western one, which then has a slingshot to the eastern base, completing the sequence. That said, even taking optimal routes, you have to move pretty quickly while defeating every officer you pass to make it work. The character the game forces you to use the first time? Oichi, who has the movement speed of a duck in molasses (though in all fairness, she isn't the worst choice to tackle this stage with. Imagine being forced to use Kanbe...)
Fist Of The North Star Kens Rage: the final chapter of Legend mode the first time through is an absolute nightmare. First of all, the game makes it very easy to collect mission stars - keep in mind collecting these gives all enemies (bosses included) an attack boost, and the only way to avoid getting enough stars for the first buff is to counter-intuitively run right past a bunch of Mooks the game lights up for you. Once you get up to the gate before the final boss (the level's one courtesy is that it's short and simple in layout), you have a gauntlet of dodging falling pillars, then it takes Suspicious Video Game Generosity to a new level by putting a two-wave ambush in the room with the stage's last pickups. The second wave of this has four minibosses at once, and if you're forced to grab the water (or worse, food) during this fight, you can end up heading into to Final Boss with no spirit and a severely drained health bar. And said boss? A Marathon Boss with no checkpoints, no recovery items present (a nasty shock, since every previous boss had a few crates lying around), capable of offing you in two or three hits with the only checkpoint being at the very start of the battle.
Chapter 2: Attack Of The Goblins in Magicka. It's only the second level, but due to a massive Difficulty Spike from the pathetically easy and short Chapter 1, it'll kick the ass of most new players. Prior to this point you've only fought a handful of enemies at any one time, but here you're constantly getting Zerg Rushed by Goblins right from the start. This is also the level that introduces Bombers and enemy spellcasters, is quite long, and ends in a Puzzle Boss that can be very finicky in multiplayer.
Chapter 5: Havindr Strikes Back could be this as well. Mainly because enemies that you've only fought briefly before are here en masse, all with special abilities such as armour, explosive ranged attacks, and magic. There's one section where you have to fight enemies on ice, with most of your attacks melting the ice and allowing you to drown. It only gets really difficult, though, when you reach Khan's Fortress, which forces you to fight constant waves of tough enemies in tight spaces while looking for levers that are extremely easy to miss. God help you if you fall into the trenches, you'll have to fight constantly spawning streams of every enemy in the level while making the long trek back to one of the handful of places where you can get back up.
Chapter 6: Return Of The Wizard. You're at World's End, surrounded on all sides by a bottomless abyss. That would be nasty enough, but you also have to fight Daemons. They're fast, hard to kill, have devastating attacks, and phase in and out of the Corporeal realm, which basically means that you have to be right next to them if you want to hurt them. They always attack in groups and split into three of the smaller type of Daemons when they die, so before you know it you'll be being swarmed with Daemonlings. The level also has very few Checkpoints, is very glitchy, and ends in the hardest boss in the game. Overall, though, this one is definitely hard in a fun way.
Chapter 8: The Mine Sweepers is a different story, though. It's very long, and Zerg Rushes you constantly with loads of annoying enemies (including Archers who can set you on fire from a range and Captains who can freeze you and block your attacks). Most of the second half has you navigating narrow pathways over bottomless pits, including one moving platform, in the dark. And it has a hard (and long) boss at the end, too.
Chapter 10: A Grim Tango. For one thing, the entire level is set on a dark plain that looks the same no matter where you go, making it hard to navigate. It's also crawling with Elementals, who are very fast, very strong, and only vulnerable to the opposite Element to the one they embody. It's quite easy to wake up more than one at once, who will almost always have different Elements, so you can't kill them all at once, especially if the Element that damages one heals another. Then there's The City Of The Dead, where you have to struggle through an army of Darksouls that just drags on for too long. The boss is also hard, but fun.
Chapter 11: Raiders Of The Lost Ruins, though, puts all the rest to shame. It's a massiveMarathon Level, complete with Checkpoint Starvation. You start off fighting Snow Trolls, who jump out of nowhere, are very fast, and can kill you instantly, and it only gets worse from there. You have to fight loads and loads of Dwarves, who are very tough, always attack in big groups, and are usually backed by their Priests, the strongest enemy wizards in the game. And that's not even mentioning the Watchers. The last section is a cave flooded with lava, requiring you to fight loads of enemies in very confined spaces with certain death on every side, including ones who can set you on fire. However, if you use Invisibility you can theoretically skip most of the fighting, but it's still tricky to sneak past everyone, and there are some sections you are forced to fight through. And if you didn't know it was there, you can very easily miss picking up Invisibility altogether.
The Stars Are Left DLC is meant as a challenge for experienced players, but most of it isn't TOO too hard...until you reach Chapter 3: The Nightmare Corpse-City Of R'Lyeh. The main reason is that the game abruptly switches from straight up action to facing you with puzzles. Of the four puzzles in the Chapter, one requires you to run at top speed through a corridor while being chased by an endlessly spawning horde of tough enemies before reaching an area where an enemy that can kill you in one hit appears with no warning, forcing you to react the right way within split second, one borders on Solve the Soup Cans, one is a more reasonable but quite tricky puzzle, and one requires you to run across invisible bridges while being harried by very powerful enemies. And once you solve two of the four, you have to fight a boss to continue. And your reward for beating that? The hardest boss in Magicka, BAR NONE.