For GDI, you have Albania, in the middle of Act 3. You have to escort a team of engineers inside an enormous Nod base and capture reinforcement bays to build an airfield. This wouldn't be that bad if it weren't for the fact that engineers die if the enemy so much as looks at them funny, and the enemy AI prioritizes engineers as the first thing they shoot. And if you lose all four engineers, game over!
All three Croatia missions are That One Level for their respective faction.
In GDI's version of the mission, you don't have a construction yard, or even enough reactors to power your entire base at once, so you are tasked with juggling power between base defenses against exponentially tougher attacks, while simultaneously fighting your way to the other side of the map and escorting an MCV back to your base through constantly-respawning hordes. And the moment that MCV enters your gates of your worn-down base, then the enemy lets loose with full force. The sanest solution is to sequence break and assault the three enemy bases at the very beginning when they are the weakest, then capture their generators to power your defenses. But even that is a task in itself, for the hardest one to take out is their Air base. It's totally inaccessable from the ground, being set on top of a cliff. If you try to mount an air strike on it, you discover that the base is full of Heroic Stealth Tanks and Missile Squads, which work with the SAM turrets to blow your Orcas to tiny pieces. The only way to do this mission is to airlift an attack force onto the small part of the cliff that the enemy can't hit with their anti-air and then hope you can take those Stealth tanks out before they rip you to pieces.
In Nod's version of the mission, you have to break through a GDI base defended by Mammoth Tanks. The only harder Nod mission, Kane's Tower, is the last mission of the campaign.
In the Scrin version of the mission, both other factions possess heavily fortified bases and will often launch heavy attacks at you in your nearly defenseless base. Even if you do fend off the attack, you will occasionally randomly lose the mission because a GDI mammoth tank trundled through the Nod base and blew up the building you were supposed to capture. Luckily, if you completed the previous mission with your Mastermind still alive, it shows up in this mission too - its mind control lets you steal Mammoth tanks to protect yourself, and the teleportation power makes crippling the GDI base a cakewalk. If the Mastermind died in the previous mission? Good luck to you.
For Nod, Operation Stiletto. Your mission is to capture two GDI Construction Yards and two Scrin Drone Platforms, in the middle of an all-out battle between the two factions. You have to capture all four Construction Yards/Drone Platforms intact, while the respective AI of both factions will mercilessly pummel each other, and usually taking down one Yard or Platform will fundamentally weaken that particular base to the point where the enemy will stomp clean through the defenses and crush you, or turn around and attack the other base which is now drastically weakened since you took out its support. Worst part is that you get almost no warning that the enemy is going to destroy their opponent's base, and usually by the time you know its going to happen, you don't have the troops ready to stop them.
The worst of the lot has to be Kane's Tower after the patches (which did wonders for skirmish and multiplayer, but more or less ignored their effect on the campaign, which was balanced for the unpatched game)—the last patch slowed down everyone's rate of income, but someone forgot to tell that to the campaign AI, which has additional independent funding. The mission was pretty bad even before the patches. It involves protecting three Scrin structures from GDI attack; doesn't sound too bad, until you realize 1. the Scrin are hostile to you, making protecting them difficult, but they're also nowhere near powerful enough to stop GDI. 2. You start out with one medium sized base at tier 2, GDI starts with two tier 3 bases, one of which is larger then yours, and the other much larger. 3. GDI will happily build not one, but two Ion Cannons before you can even begin construction of your own Nuke. 4.GDI starts with a fully functional small airbase, which will bomb the Scrin to the ground if not disposed of 5. Your two starting tiberium fields are both in harms way, one from the Scrin, the other being the main avenue on GDI's attacks on your own base. 6. GDI is shelling the structures you're supposed to protect from the second the mission starts with unique artillery that out-ranges normal artillery by a factor of 5—capture-able sure, but only if you manage to get to it while simultaneously dealing with all the other difficulties at the same time with your very limited early game resources. Winning the mission often depends on luck more so then anything, as the GDI attacks on the Scrin vary in size, from manageable, to an armored column consisting of a dozen mammoth tanks supported by twice as many lighter vehicles and about as many infantry squads, as well as every single reinforcement support power in the GDI arsenal. Losing (which will happen very often) will earn you a chewing out by Kane himself, despite the fact that his request to protect the Scrin structures is completely unreasonable.
One particular late game Nod scenario in Command & Conquer: Tiberian Sun tasks you with finding a UFO with Kane's Tacitus inside. You are given three light infantry, three engineers, two light buggies, and one attack cycle. There's a sizable GDI presence on the map, including a couple of Titans that will catch up to you if you pause for too long and will annihilate you if they do. After you sneak your way around the map to the UFO, you find that Vega snatched the Tacitus and have to stop the train before it leaves. You sneak back around the map to Vega's base, and have to dash past his defenses and pray you've got enough oomph left in your tiny force to destroy the train before Vega's men either catch up to you and finish you off. Oh, and if you're too slow to reach the train, an engineer repairs the broken bridge, and the train leaves. If you lose your attack cycle (and might I add, attack cycles are the epitome of Fragile Speedster), then chances are good you won't be able to stop the train.
The original Command & Conquer had an mid-game GDI mission where you have to protect a village. The problem? The villagers wander randomly, and there's a tiberium tree which will poison them when near it. You can't evacuate them via APC, either. It appears that it's supposed to be a Timed Mission, but the random wandering makes it more of a Luck-Based Mission if all the idiots decide to go breathe in the delicious gases. Thankfully you can choose another mission instead of this one.
Let's not forget the GDI mission 11 from the original Command & Conquer that had your team hitting the beach in an effort to raze a Nod base in the area. No, not the first mission. The one later, where your infantry team has to clear beach turrets to make room for an MCV to land. Then you have to protect the MCV as you get to the spot where the tiberium is, all the while armed only with infantry. The best part is that you have about 3 minutes before an enemy flame tank—which eats infantry for breakfast and structures for lunch—comes rolling into your base, and unless you have your refinery in a pixel perfect place to maximize the rate of income, you probably don't have any vehicles by that point, causing your torched troopers and scorched structures to be the last thing you see before you reload your saved game, repeatedly.
Then there's two missions from 'Covert Operations' bonus disc. C&C: Death Squad and C&C: Under Siege. First one includes you playing as NOD, you have to destroy GDI's Adv. Com. Center with following units: Two Stealth Tanks, Flamethrower Tank, two Minigunners, one Bazooka Infantry and Commando. Problem comes from that GDI haves fully upgraded base and can replace their lost units. Latter one includes you playing as NOD, having fully upgraded base, you're surrounded by cliffs having only three exits. However, there's loads of GDI units on each exit and you don't have any money with you. GDI also likes to use Air Strike in your base fairly often.
Command & Conquer: Red Alert has the second to the last Allied mission taking place indoors where your limited number of infantries are split into two teams. You have shoot hordes of enemies while your engineers to disable the controls and there's a time limit.(sorry, no Tanya this time)
Command & Conquer: Red Alert 2 penultimate Soviet Mission. The enemy can attack rapidly, mind-control your units, and is highly resistant to artillery attacks (since your artillery units' projectiles get shot down). If you successfully turtle, you can nuke the objective building, and there's also a few passages that you might not notice because you're too busy trying to turtle.
Command & Conquer: Red Alert 3 has the last Allied mission. Your objective is to destroy seven strategically placed Iron Curtain machines, which make the Soviet Premier's palace invulnerable, and then take down the palace. Every one of the Iron Curtains is in the middle of a Soviet base except one, which is in the middle of an urban area whose all apartments are garrisoned full of Soviet infantry. This wouldn't be that much of a problem were it not a Timed Mission: On easy you have an hour, on medium and hard it's thirty minutes. The island you and your co-commander are given as a base location is pretty easy to defend, but has only four ore mines and two oil derricks, a pretty small amount compared to most maps in the campaign (considering you probably are too busy taking down the Iron Curtains to clear yourself a way for a secondary base to increase your resource gathering speed). The enemy begins by sending huge tank rushes, which are easily countered. Then, when about fifteen minutes of your time has passed, they suddenly send an enormous infantry assault, just in case their lack of such attacks has made you forget to build anti-infantry defenses. About the same time, the Soviets start attacking with V4 launchers (which in this game can't be countered by shooting the missiles, so you are required to have units to take them down before they can reduce your base into shreds) and their superweapon activates. Five minutes later, the soviets start to bring Dreadnoughts against you via the river and attacking you with helicopters and zeppelins. At this point, even the hardiest Allied general is reduced to a crying pile on the floor.
Ditto the final Soviet mission. While it is not timed, you have to put up with massive enemy forces. You have to worry about aircraft carriers and constant airstrikes bombing you to oblivion, paradrops right behind your base, and if you think it couldn't get any worse, the enemy will start using the chronosphere to drop vehicles at undefended parts of your base, and also bring a Proton Collider online. Not to count your co-commander often did a poor job at fending off the Allies navy.
These two missions are easy compared to THAT one Empire mission. Remember the one you have to fight off both Allies and Soviet commanders with Shinzo as your co-commander? They are across a stretch of water, so land vehicles are out of the equation. Send amphibious units or battleships? You might be able to destroy about five defense emplacements and about two power plants close to the water, then watch the ships break apart. Or get turned into the world's biggest and most fragile piece of ice. Or get sucked into space, never to be seen again. Maybe. You better hope not. Or... Anyway, you finally decide to send air units. Great. See THOSE funny looking trucks with the massive cannons and twin-barreled guns? Or those unassuming cars which fire off a Macross Missile Massacre? They are also fast enough to run away before you get close enough. On the hardest difficulty, it takes so much planning and micromanagement to make an Allied commander proud. And you have to (strategically) sweep their base aside before you lose enough units. Because while you are building another wave, they are building another base. And about the only use of Shinzo (for any other missions too, for that matter), on the hardest difficulty, at least, is as cannon fodder. Seriously. Try asking him to 'hold position' between you and the enemy.
Command & Conquer: Generals - Zero Hour, General's Challenge Hard, Alexis Alexander. You start with the standard-issue command center and single construction dozer on a small island in the left bottom corner of the map. Alexander has a sprawling base covering most of the opposite corner and a ring of missile batteries in the archipelago between the two bases. If you make the slight mistake of building anything on the western side of your island at the start, Alexander destroys whatever you were building with a triple A-10 airstrike and sends a trio of Comanche choppers to whittle down your command center 30 seconds into the mission. This is way before you even have a barracks built, much less base defenses or anything else that can answer to aerial units. Aside from that little curbstomping surprise, the entire mission is basically about exploiting the AI's tendency to act on invisible triggers. If you can't trick the AI to waste its Particle Cannons on one or two cheap structures in the same spots every time, you are going down.
Don't forget that said missile batteries are EMP missile batteries, and will shut down vehicles they hit. If said vehicle happens to be a flying vehicle, bad things happen...
Alternately, you can play her tricks against her. Assault the one P-Cannon that's on your island (but don't destroy it). Use one of your abilities to knock it offline (to prevent her from selling it on you) and immediately capture it. She'll voice a complaint about your use of "lack of sportsmanship", then shoot her particle beams at it. To really play her good, initiate the sale of it about 8 seconds before she shoots, letting her waste the shots and getting $2500 from it. One P-cannon down, two to go.
There is also the 3rd Chinese mission. Your base is on the outskirts of a huge, sprawling city absolutely infested with the GLA. Your objective is to destroy all of the statues in the city before your 'international opinion' drops too low. On hard difficulty, this mission is the biggest nightmare imaginable even to a veteran player. The GLA has stinger sites, tunnel networks, and demo traps absolutely littering the city. All of them are stealthed, so the stingers and tunnels will almost always get the first hit on you and the demo traps will destroy entire tank columns if you don't advance very carefully. Meanwhile, the GLA is sending nonstop waves of rocket buggies and SCU Ds at you. These units are much, much, MUCH faster and longer-ranged than anything in your arsenal for this mission, and they're smart enough to retreat as you slowly charge at them. A typical scenario might be a SCUD firing on a tank column from an entire screen away and instantly killing or crippling half of it. As the tanks pursue, the SCUD falls back. Determined not to let the enemy go, the player orders them on... and they trigger a demo trap which kills everyone. If you instead let the SCUD go to avoid the risk of falling into an ambush, you can expect it to be back as soon as it has had time to reload.
Prince Kassad. He has TWO bases along two different routes of the map, both guarded by stealthed, mutually supportive stinger sites, tunnel networks, and demo traps. Once you destroy a stinger site he sends a worker to rebuild them ALMOST IMMEDIATELY and the construction site becomes stealthed after a few seconds. He continually sends rocket buggies, technicals full of terrorists, and demo trucks to harass your base from TWO directions. There is an oil derrick and reinforcement pad to the North, but the land route is protected by demo traps and you still have to deal with stinger sites and a tunnel network. If any mission requires you to just turtle and unleash six or seven superweapons, this one is it.
He loves to use the Sneak Attack, the Rebel Uprising, and the Anthrax Bomb attacks. The Sneak Attack is countered by a handful of tanks kept near your Command Center; the Rebel Uprising by widely distributing infantry-mowing Snipers or Gatling Tanks; and the Anthrax Bomb aircraft always follows the same path, so load it with AA weapons to kill the bomber before it can drop. The Scud Storm will fall to just about any combination of two high-damage support powers.
Also in the running is General Shi-Tao "The Nuke." He has up to five nuclear silos, all of which activate within minutes of the start (guaranteeing you don't have the capability to take them out, even with the generous twelve minute timers. The area around his base has spontaneous radiation fog, and he keeps a steady stream of nuclear powered tanks, nuclear artillery, and infantry to your base. Fortunately the infantry are near death anyway, but the artillery will blow up just about any structure in two hits. Oh and since the projectile is a shell, it can't be intercepted.
The (usually) penultimate, General Townes' level contains stealthed observer drones that somehow ended up under the ramps to his stronghold entrances. These are impossible to kill, yet all your AA guns will immediately target it upon detection. This would not be a problem except for the fact that he constantly sends attack choppers to engage your assault groups. Because your AA guns are automatically targeting the drones, guess what they're not shooting at.
General Leang has the best picks of all weapons available to all three sides to suit a refined tank assault style (She gets Paladin tanks, Overlords, Rocket Buggies, all three special characters, all three superweapons (timed to fire simultaneously) and a sprawling base studded with gatling guns and patriot batteries fitted in places unaccessible to normal range tank guns. She can also use tech combinations that are impossible for the player, such as Chinese Overlord tanks with the U.S. Composite Armor upgrade. She WILL make you fight bitterly for every inch of ground. However, her superweapons are all on generously long timers, which can be reset by destroying one of several poorly-defended structures scattered around the map. She also doesn't have any offensive support powers. This means that whereas previous generals were constantly harassing you with artillery strikes, A-10's, anthrax bombs, and superweapons, the only thing you really have to deal with when fighting Leang are her units.
In "The Hammer Falls", the final mission of the Terran campaign, you're getting pounded from two directions by enemy Terran forces with all of the same units you have, your base is surrounded by cliffs so you can get attacked by units you can't see, and your starting area is quite small, so if you aren't smart with your building placement you could end up with no room to build. The enemy has no qualms of doing stuff like irradiating your SCVs, bombarding your base with siege tanks, or having ghosts lockdown your mechanical units, which are the backbone of the Terran forces. And that's not getting into the nukes, which will blow apart your bunkers, missile turrets and tanks, leaving your defenses crushed in an instant, and because ghosts can and will use nuke from outside the range of your missile turrets and siege tanks, even if you find them you probably will not be able to move fast enough to kill them before the bomb drops.
The Zerg campaign mission "Eye for an Eye" of has you trying to hunt down Dark Templars on Char. There are several exits on the level, and they'll make a break for it if you ever move a single Overlord out of position, or leave it unguarded by regular troops. And if a single one escapes, mission fails. Thank God for level-skipping cheats.
Far worse is the Protoss civil war mission "Homeland". The enemy Protoss forces have every unit you do (except your extremely limited supply of Dark Templars and the special characters which you can't afford to risk) plus they get the dreaded Carriers and Arbiters which you DON'T get. They have two large bases and massive supplies of reinforcements which can crush any attack force you send. The saving grace of the level is that a backdoor entrance to their base lets you use the Dark Templar, who are invisible normally, to infiltrate their base and destroy their Nexus. It's a bit of a Guide Dang It though, since said backdoor is way down at the very bottom of the map, you don't know it's even there or that it has no detectors so the Dark Templar can get inside safely, and the mission objective "destroy the heart of the Conclave" doesn't state it means specifically to destroy the Nexus, so you're liable to think you have to level their entire base like most every other mission.
The penultimate Protoss mission "Shadow Hunters" gives you 2 heroes that have to survive the mission and a small strike force with a few probes to build up your base. Your gas and minerals are miles away from each other, and Zerg start attacking very quickly. The zerg also regularly send out defilers to cast plague on everything in your defence which deals up to 300 damage to everything it hits ignoring your shields, but luckily can't actually finish anything off, leaving them at 1 hit point. That includes your heroes, which you simply can't not rely on for defense in the first 15 minutes. Once you think you have a decent defense up, and are considering going on the offensive, you'll get bombarded by Guardians from a safe distance, while armies of Mutalisks prevent any anti-air from actually going to the Guardians. If you manage to hold all that off, then congratulations, you can finally start slowly making your way through gauntlets of Zerg all the way to to the 2 main bases where one of your heroes has to deal the killing blow to the Cerebrates. But if you survive the first 20 minutes, that's rather trivial. If.
The last 4 missions in Brood War are notably harder than the rest of the Zerg campaign:
The seventh Zerg mission "Drawing of the Web" requires you to bring Duran to 5 beacons. Duran is frail, his cloaking is worthless near the beacons thanks to omnipresent detectors, and it's an automatic game over if he dies. Each beacon is on its own separate piece of elevated terrain, with one ramp in/out, and a small base surrounding it. The elevation gives the defenders a damage & armor bonus against anything approaching from below, and there's a large, untouchable Protoss base at the center of the map providing reinforcements. Overlords can't transport units in this mission and you don't have access to Mutalisks, making the terrain problem unavoidable. Attacks on your base start early, hit hard and never let up; Archons will show up in the early attacking waves.
The eighth Zerg mission "To Slay the Beast" is no picnic either. The enemy is a combined Zerg-Terran force, thus they have access to every unit you do and can combine it with the Terran army for extra efficiency. Late in the game you can easily get swamped by a fleet of Mutalisks, Wraiths, Valkyries and Guardians, which will absolutely slaughter your own air units, and without Dark Swarm to protect your ground-units they'll slaughter them too. It doesn't help that your objective is on the other side of the map guarded by two full enemy bases, in addition to the full dual-base in the middle of the island and the two bases to the north and east of your starting position. You can have some fun in this mission though by taking advantage of being able to place Nydus Canals on enemy creep, thereby allowing you to clear a back entrance to their base with Guardians, then build a Canal there to instantly transport your ground forces to the hole you punched.
The ninth mission "The Reckoning", which requires destroying every. Last. Protoss before they escape in a set time limit, while fending off Terran attacks on your base. This is difficult on its own, but you have to do it with 5 minutes to spare for the bonus level unlock.
The final Zerg level, "Omega", has you pitted against two Terran forces and one Protoss force. They each have massive amounts of units that work especially well against the Zerg and require different strategies to handle each, with well fortified bases and organized, sometimes concurrent, attacks on yours. Each of the forces is so dangerous on its own that your base can be overrun even after you have wiped out 2 of 3 opposing armies. Resources are finite; expanding your resource gathering operations beyond the starting base is practically required, but the attack on your new hatchery will be so fierce that you'll spend most of your new resources replacing units lost defending it. You could apply damn near half of the CVGSY library to "Omega" and it would still be an understatement.
The Protoss mission "In Utter Darkness" is a weird example, since it can only be lost if you intentionally try to do so. Your only win objective is to kill a certain number of enemies: once you meet that quota, you can die and the mission will count as complete. The catch is that the bonus research objectives require you to hold out for 25 minutes and to destroy a certain amount of enemies. The enemies come in massive numbers from three directions, are accompanied by Elite MookHybrid units in increasingly higher numbers as the mission progresses, your walls are regularly attacked by mutalisks and brood lords, and later you're regularly attacked by nydus worms unleashing waves of broodlings on your base, and they can pop up anywhere so you need to plant cannons all around your base out of paranoia. The common strategy for this mission is to wall off the base entirely and rely on air units and siege units to kill enemies from safety, but even then you need a lot of micromanagement to focus down the enemy air units and mentioned Elite Mooks before they bust those walls down. This mission also suffers from a greater Difficulty Spike than other missions on higher difficulties, because the amount of enemies you need to kill for that bonus objective increases, in addition to the fact the enemy forces come in greater numbers and upgrade their stats quicker. This is the only mission where the achievements (going over the kill score by certain amounts) can all be done on Normal difficulty, and good thing because you'll have hard enough time meeting the base requirements themselves.
Hard mode "Zero Hour" is only the game's third mission, and is a Wake-Up Call Level for the rest of Hard mode. You have to hold your base for a half an hour, and this is quite easy on Normal. On Hard however, the zerg are more aggressive, they call down pod drops to spread creep to increase their movement speed when they attack, they destroy the neutral structures around the map to deny you vision of their movements, they will drop creep on your base unleashing zerglings, and of course they attack more aggressively and in greater numbers. The achievements for this mission require you to destroy 4 Hatcheries on Hard mode, and going on the offensive is not easy because all you have are medics and marines, and the zerg bases are on high ground, forcing your units into a bottleneck while the zerg defenders swarm the top of the ramp to push them back. And you have to time your attack too, because if you move out while a wave of attackers is preparing for an assault, guess who's going to get caught in the open and devoured.
On Normal mode, the campaign is pretty tame, and even on Hard it isn't too tricky if you're familiar with the game. Then you come to the final mission, "All In". It's a Hold the Line mission where you must defend a special building until it charges up energy to 100%, the mission lasting over a half hour, during which you fight off waves and waves of Zerg trying to destroy it. This is hard but not overly impossible, until you factor in that you face a Recurring Boss unit that can kill most units in two or three hits, and has two spell attacks, one that does heavy area of effect damage, the other of which instantly kills any unit. Oh, and this boss unit has a lot of HP to boot. The level also has a unique gimmick depending on which of two missions you did beforehand—you can either deal with Nydus Worms regularly sending out additional waves of enemies to attack you, or you can face waves of Mutalisks, Corruptors and Broodlords who come with a second Boss unit later in the mission. The mercy of the mission is that the building you're defending can regularly put out an energy wave to kill all nearby enemies. You will definitely be needing it, and for fun, one of the Achievements is to win the mission on Hard and only use this energy wave ability once in the entire mission. Good luck.
The aptly named Brutal Mode can turn almost every mission into That One Level, as enemies are much stronger and much more competent, in particular utilizing area of effect-damage units (Banelings, Ravens with Hunter-Seeker missiles, Colossi and High Templar) to great effect. They also get access to much stronger units than they have on regular difficulty, such as Banshees and Ravens on "The Great Train Robbery", a mission where depending on the order you're tackling them, it's possible that your only source of anti-air power is the basic Marine. Said mission requires you to destroy a series of trains, which have several thousand HP and come with garrison units to protect them. On all other difficulties, the Dominion sends out a Marauder team to patrol and defend, but on Brutal they send out two teams. The Marauder is a beafy anti-armor infantry unit that slows down units it hits, too bad the ideal unit for this mission that you have to build in great numbers to win is an armored unit whose strength is the ability to attack while moving. Siege Tanks can help whittle down the train's health and take out its escorts including the Marauders, but even they can't defend against the Hunter-Seekers.
"Engine of Destruction" can be tricky if you don't know what you're doing. It's an Escort Mission with a Leeroy Jenkins hero that will barge into enemy bases with no sense of self-preservation, and it's up to you to keep repairing his unit to keep it from being destroyed. Thankfully this hero is a One-Man Army and actually is strong enough to take out those bases on his own like he's trying to, but you're still liable to find yourself screaming at him for not giving you enough time to repair him before he goes charging in again. Also fortunate is that after this mission comes "Media Blitz", which is a good contender for the campaign's Crowning Moment of Awesome.
"Supernova". Dear God, "Supernova". You're on a planet that has an advancing wall of fire going from left to right, and which will rapidly destroy any unit or building it touches. This means the player is forced to move the main base constantly to outrun the wall of fire (preventing said player from building up a substantial attack force). Add to that relentless attacks by the Tal'darim protoss, and some of the best bases being well-protected by said Tal'darim... well. And the player can't dillydally in acquiring the research points, lest the fire get to a protoss artifact before the player's units do. Thankfully, for once you don't have to build Supply Depots for your army, or this level would be far worse than the final mission.
In the 11th mission in Dungeon Keeper, you have to defend your dungeon from wave after wave of heroes, coming in four directions. You do get some time to prepare, but plan wrong and everything will go awry. Fortunately after that, everything is much easier.
Although reportedly every level in Dungeon Keeper has a "Transfer Creature" item (allows you to take one of your creatures through to the next level), so with some experience of this you could bring through a level 10 creature to help.
"Batezek" from the Deeper Dungeons expansion takes everything that made Hearth tough and sends it straight through the roof with groups of heroes coming in at all directions. If you don't wall yourself in, the stronger heroes that the tunnelers will release at one point will destroy you unless your minions are somehow maxed out in level.
World in Conflict really loves to abuse this situation. Only it takes it a step further. Since the way you control territory is by means of control points, not only do you have to defend up to three points, you have to press on an offensive elsewhere. This wouldn't be bad if you had more than 4 tanks and 6 squads of troops, and you need about 1 tank and 2 squads minimum to hold a spot for a respectable amount of time.
One of the worst ones is in Company of Heroes, mission 12 and 13. Your main objective is to defend not one, but two locations: Hill 317 and a little town nearby called Mortain. The first part isn't too bad, it's mostly driving the Axis out. The second part however, will really test players, as if the game wasn't hard enough up until then. Not only will you be pounded from every direction on Hill 317 (and you have to defend Mortain), but the Axis will pound you with mortars and 88 flak guns. And all you're given is whatever you were able to save from the last mission plus a howitzer.
Another terrible defense scenario is the Hard difficulty version of the mission "Reinforcements," from Warhammer: Battle March. You get attacked from multiple directions by waves of leveled-up, angry Chaos units that outnumber you by a ridiculous margin - so that you can recruit one tribe of Orcs to your cause. The final wave is especially hellish, as the Chaos units rush you with hordes of maxed-out heavy cavalry and monsters.
Empire Earth has the second Novaya Russia mission, where you're surrounded by enemies at a higher tech level (read: they get giant killer robots and have little in the way of ressources, so you mostly end up having to protect your workers from attack.
In Evil Genius, there's one mission in which you need to steal 25 crates of technology in order to build your Doomsday Device. Each mission takes only 15-30 seconds, and repeat indefinately until you personally stop it. What makes it so difficult is that this indefinately-repeating mission has a high Heat rating, meaning even stealing one of these crates drastically increases the likelihood that the Forces of Justice will send shoot-first-ask-questions-later Soldiers and Veterans to your Elaborate Underground Base. Either you spend hours in real-time stealing one crate, waiting for Heat to die down, and stealing another one... or you run multiples of this mission in different areas of the globe, and pray that the game remembers that you made each Force of Justice hostile to each other in an earlier mission... which it has a habit of forgetting about about half the time.
And heaven forbid you're trying to pull off one of these operations in a region of the globe where a Super Agent is still active. Starting any missions while a Super Agent is active on the world map draws them like flies, and when they arrive they'll start whacking your minions like a chainsaw through saplings.
Impossible Creatures has 'The Island of the Crazies'. Basically, Rex gets infected with a virus that'll make him crazy THEN dead and has to destroy the defences around the sole container of antidote on the island and move him onto it within 15 minutes. Also, you creatures have the virus, too, but it takes less time for them to go crazy (rendering them uncontrollable). Take into account that you need to go get some new DNA to help you out, and the length of time that building a base, making creatures, gathering resources, advancing research levels, etc., requires, and...well, not much time left.
Para World has Mission 15: New World Order. Quite literally, it's spears against machine guns. Unfortunately, you've got the spears. And limited resources. And enemies on either side. And a timed mission which involves capturing five pumping stations. The timed mission doesn't seem so hard until you capture the third station, whereupon masses of enemies spawn, and two cannons appear that do 500 area damage in one hit.
Don't forget the mission one or two before the final one, that prison break mission. I gave up the game for months because of that mission. If you defend from the land attacks, they hit you from the sea (the tip that allowed me to win was "sink the ship making the transport vessels"). Oh, and did I mention those fucking robots? Pretty much every SEAS unit carves through yours like paper, including your defensive walls. Ugh.
From the boosterpack missions, the very first of them will have you screaming in frustration. Basically, you're caught between two opposing bases, and you have to build your own, take both bases out, and transport a set amount of units to a set point within a time limit. As soon as you start building your base, the enemy forces will swarm through on either side and destroy you. Oh, and the route to the point is lined with vicious dinosaurs. Have fun, kids!
There's also 4-10, a level that's completely in the dark, illuminated just briefly by flashes of lightning. Considering you have the dreaded Digger Zombies, Balloon Zombies, and Pogo Zombies all to deal with, it's nightmarish enough. But the kicker? It's a conveyor level. Good luck with that.
Several of the strongholds in later expansions count, but a special mention must go to the Tau stronghold in Dark Crusade. For starters, their early-game rush is much, MUCH stronger that any of the others in the game and can outright kill you on harder settings if you aren't properly prepared. Secondly, the map is completely mobbed with Fire Warriors that start with all of their upgrades. Finally, there two command posts at opposite ends of the level that start spamming high level units if left for too long. Merely having one of them start makes the level really hard. If both of them start producing units, the level becomes practically unwinnable.
Orks don't enjoy the Chaos base very much either. Why? An Ork Trukk can hold one unit and takes up 3 vehicle slots. There are lots of pillars that pulse out infantry-killing blood shockwaves every so often, meaning you have to hide your infantry in vehicles unless you have really good timing.
The Necron stronghold will trip you up if you go there before you have anti-vehicle Honour Guard to kill the Tomb Spyders while your other men fight off the initial Necron rush. That's before the rest of the funky effects scattered around the level kick in unless you destroy the Necron beacons first.
A non-Stronghold one is the one to unlock the "Advanced Base". When you attack this land, you have no base. When the Orks control the territory (which is the case by default), they can level your force (even with a full honour guard) by sending a tide of flash gitz, looted tanks, and a new Squiggoth as soon as the current one dies.
Every race starts with a powerful force when assaulting the Hyperion Peaks, typically even including Tier III infantry and vehicles along with a super unit or two. Every race, that is, except the Tau Empire. You don't start with a single squad of Fire Warriors unless they were in your honor guard. Their super unit, the Greater Knarloc, is a slow oversized Komodo dragon with no ranged attacks that typically spends most of its time turning around. You have two Hammerheads and a couple of stealth suits to take on about three WAAAGH's worth of armor.
Chaos doesn't get much better for that mission as well. The biggest problem with this mission is the fact that you get no Chaos Space Marines or Khorne Berzerkers (The latter being at least unobtainable without Tier II). Instead, you get some Possessed Marine squads, which would be excellent if it weren't also for the fact that you also have to work with a mob of puny cultists as well. For the record, cultists are pretty much useless without new weapons or at least a champion, and you can't use any honor guards to attach them. The only real benefits you get are the vehicles (3 Predators and Defilers) and the Daemon Prince, your mightiest unit ever, which can at least heal itself.
Every single stronghold mission in Soulstorm is That One Level. Tau can attack anywhere on the map with a ridiculously powerful cannon and, like the Orks, possess multiples of their most powerful units (their numbers are supposed to be limited to one or two). Imperial Guard can build multiple Baneblades if their factories aren't watched carefully. Space Marines frequently send fully-upgraded squads directly into the player's forces. Both Necrons and Chaos can spawn their two most powerful units at a moment's notice. Eldar have half a dozen hidden bases, each with massive rushing potential. Dark Eldar only allow a base to be built halfway through and spam their special abilities. The Sisters of Battle stronghold is the worst though, possessing an invincible unit that causes everything else near it to become invincible as well. The only way to defeat it is by destroying four icons: said unit immediately teleports if one of them is under attack.
The only way to win the Sororitas stronghold mission is to deliberately take massive casualties. You mount simultaneous attacks on two of the icons, allowing the Living Saint to tear one of your forces to shreds while the other one levels the icon. When there is only one Icon left, you have to throw a large force directly at the main Sisters base to draw the Saint away, and then hit the last Icon with a small demolition force that you kept back. And sometimes even this tactic does not work, because the Saint refuses to go away from the Icon.
Halo Wars level ten. Shield World. Lets recap. You have ten minutes to save three squadrons of men (who by the way, are absolute nobodies apart from three SPARTANS). Your base is in the least helpful location, you don't have time to build up the forces necessary to really take on the enemy, so your only option is to spam low level units which you can't make enough of because of the mind-numbingly stupid headcount. The only way to increase your time is to EMP four pylons, each of which grants you five minutes (which makes no sense, since they are the only thing imposing the time limit, so with all of them gone, why the time limit?) The enemy get constant refills of forces, and since your fighting the flood, anyone you lose becomes one of their number. And the barriers you have to destroy have magically respawning health, so you have to take them out in a single raid, while being pounded with artillery fire, air units, and ever increasing ground units. Not for nothing is this the level which made Yahtzee rage quit.
That said, Shield World isn't as nasty as most other levels on this page. At the risk of going into Walkthrough Mode, the level is made much easier by teching up first, blowing up the supply pads, and spamming gauss warthogs/marines preceded by cryo bombs.
Majesty 2: there's a level in which your castle is trapped between two camps of enemy heroes. On one side, endless rogues and elven archers that can slaughter your heroes due to poor anti-archer AI. On the other, paladins with healer support. Your only chance is to get lucky enough to kill early enemy heroes before they level up.
The archers aren't too difficult. They're weak. It's the paladins that are horrible. Once they level up, they get insane dodge, health potions, and everything. There's basically two ways to win. South and east are safe, so build in that direction. Either use a house of lords ranger to destroy the paladin's marketplace early (if you have one level 35+), or tower up heavily. Otherwise, you either die or get the fun fun opportunity to have a brutal multi hour slog of attrition.
Let's not forget one of the early missions where enemy wizard attacks your base all the time, damaging all your buildings unless you get level 2 wizard tower. Oh, and by the way... There's chance that level 15 Werewolf will come in your small-not-prepared-for-such-thing base and destroy it. There's also that it does 100 damage per hit, killing normal, level 1 heroes with one hit.
Hostile Waters: Antaeus Rising: In the level where you return to island zero, If your first wave fails to cause crippling damage to the bugs' main production facilities, it becomes almost impossible for you to collect enough energy to clean the island. But the bugs, of course, have no similar problems.
Also the isthmus mission. Recycle everything. Create a wave straddling the isthmus. Inch forward. Create carnage, a little bit at a time. Advance to protect the recycling units. Use almost all the recycled bits to replace thrashed units. Repeat oh, maybe a dozen times. Then hope there's enough energy left for your main assault.
If you're a Rebel and your opponent plays the "Defense Star" tactic, putting a Death Star with shields and a sizable fleet with a lot of TIE Defenders over Coruscant, preferably with some Interdictors just for fun, your fleet will be destroyed. To explain, Interdictors prevent a jump to hyperspace, TIE Defenders are the most badass starfighter in the galaxy, and if you have Death Star shields, you can't lose. Finish this up by stocking the Death Star with stormtroopers to prevent a sabotage attempt. Yes, if you've survived long enough to get TIE Defenders, it's easy to drag the war out as long as you need as the Empire.
If you're not playing headquarters-only, capturing Luke or Vader can be extremely difficult because of their damn high Combat statistics. It gets worse for the Empire if Luke's been on a lot of missions; every successful mission gives him "Force points" which he can use to increase his level and with it, his stats.
In Warcraft 3, Into The Realm Eternal. An early Undead mission, the prime reason for it's frustration is that you have three units available to you. Ghouls, necromancers, and the meat wagon siege weapon. The limited troop types means that a balanced force is impossible, and there is only on strategy available to you; the Zerg Rush. And even by Zerg Rush standards your troops suck. Ghouls are fragile and don't deal all that much damage, meat wagons are slow and fragile, and so are necromancers. Their ability to Animate Dead is vital early on, but the computer knows that the troops they summon will fall apart after a while, and doesn't bother to attack them until all the things that matter are dead. And when the priests start to show up in large numbers their dispel magic spell can wipe out swathes of animated skeletons in seconds, as well as buffing their troops and healing them through ghouls' attacks.
Forget that, the true challenge is the Night Elves scenarios. Namely, the fourth one: with only a few troops, you must move your base along a twisted, very long path with tons of obsatcles in your way (including feral Furbolgs, a whole human base and a wood which is totally filled with undead monsters that respawns untill you kill their chief (a high-level wraith)and include mainly Banshees, who have the nasrty habit of possessing your soldiers when their health is low. Oh, and you must also look out for casual Orcs and Undead skirmishers. Seriously, this part is far worse than the last level.
The respawning monsters can actually be a bit of a blessing really, because you get gold every time you kill one of them and your two level five heroes can burn through a pair of respawning level three skeletons time after time after time at 44 gold a pop. Banshees do pathetic damage to heroes and can't posses them, so if you're willing to take your time you can refill your coffers by fighting those monsters over and over again with your heroes.
On normal difficulty, the missions will generally not be too much to handle for an average RTS player, but once Hard is enabled, some missions require more creativity and thoughtful expanding, to put it lightly. To avoid swarming the page with entries, some notable stand-out levels on hard mode are as follows:
March of a Scourge: The enemy applies considerable pressure, attacking with a much stronger army and siege weapons to makes your towers unable to Hold the Line on their own, and they send a lich hero to "death and decay" your towers and nuke your army for good measure. Base expansion and multi-tasking skills are suddenly required in a campaign that has not been too difficult up until this point.
The expansion pack The Frozen Throne had a number of teeth grinding levels, however the one which stands out is in fact the very first Undead level "King Arthas". In it each of your three heroes is tasked with holding a different path off the map from escaping humans, whilst also advancing down each and knocking out their camps. These camps are protected by teleporting high level paladins, making each an involved fight- whilst escaping humans trickle down the other two paths and if allowed to the friendly AI is aggro'ed into allowing them through. You can't build any defense towers of your own, each of your heroes can only construct one or two types of unit for themselves, and needless to say if too many humans escape you automatically lose. Even the very last level seems less annoying in comparison.
Homeworld 2 has some notoriously difficult levels, due to the fact that at the start of every mission, the AI gets a fleet several times your own which also happens to have the perfect counters to your ships. In fact, hacking the game files and giving yourself more ships than your quota would allow will backfire since this will increase the size of the enemy fleet EXPONENTIALLY. Never mind that the AI ignores unit limits anyway.
Two Words: Thaddis Sabbah. As mentioned above, the scaling enemy fleets can make parts of the game ridiculously difficult, but this level takes it up to eleven. Your objective is to rescue a prisoner from a space station. The fleet guarding said station at least outnumbers your own three to one. You can only have two battlecruisers at the same time - your enemies will have seven - and as mentioned on the Homeworld main page, their fighters will possibly outnumber every fighter you've built in every mission in the entire game combined. If you're unlucky, you could easily spend something like a third of the total playtime on a playthrough trying to beat this one mission. Out of 14. The difficulty was nerfed in a patch, with good reason.
God have mercy on you if you play Europa Universalis III as one of the indigenous North or South American societies, even if you start in 1399. You do have some breathing room until the European countries show up on your doorstep, but you start out extremely poor, with no forts in your provinces, with your slider far on the decentralized end of the scale, and with your research way behind most other countries in the game. You might stand a chance if you manage to Westernize, but that requires that you survive for a fairly long time and have more than a bit of luck. Better hope that in your game the Ottoman Empire invades at least most of Europe!
Hell, just playing any small country with a much larger, stronger neighbor is a huge challenge that seems to require lottery-winning luck more than skill. For instance, if you're playing Scotland, pretty much your only chance of just lasting to the 1800s is if England is invaded, breaks up, or your dynasty inherits their throne. No wonder EU III uses a skull and bones to denote the hard end of the challenge scale in the start menu...
Or playing as any country that neighbours a Horde country in the Divine Wind expansion pack. These horde countries will always go at war against you every time their truces expire, and will always field armies that will be always larger, stronger, and better led than whatever you can throw against them. A notable example is Georgia, which shares borders with three of these countries, and all of them will mercilessly destroy it until it gets erased off the map. Really, the only chance to beat them is to go through the land technologies as fast as possible to be able to beat them.
Dark Reign has mission 5, which is infuriating no matter whose side you're on. The Freedom Guard get a hopeless level where they have to save up 30000 credits, while holding off waves of Imperium tanks. Wouldn't be so bad, if the Tachion Tanks didn't make their debut appearances in this level. As a result, the only viable tactics are to constantly build units, meaning you'll never hit your goal, or gather up everything you start with, bring it back to your base, and only build when absolutely necessary, meaning you'll get near your goal but get stomped by Tachion Tanks. As the Imperium, you have to destroy the Freedom Guard while protecting a MacGuffin located inside their base, which they attack the moment you show up on their doorstep, meaning the only possible way to win is to build a horde of Tachion Tanks and pray you can break down their defenses quickly, destroy the units attacking said MacGuffin, and then hold out long enough to build some reinforcements and get them up there to mop up the rest. Oh, and the Freedom Guard get access to Martyrs in this level, meaning taking your eyes off your base for more than a minute means you'll find smoking holes where all your buildings used to be. While the rest of the levels after this get harder and harder, this one takes the cake for sheer frustration.
Or, you could look at level 11. As the Freedom Guard, you have to move your base and find a suitable location to establish it in (thanks to malicious spring placement, easier said than done), and wipe out the Imperium. It'd be fairly standard if the Imperium didn't get access to a Rift Creator, which will tear anything it's being fired at apart, and if you didn't have to protect Jeb Radec, who has a habit of attracting Shredders, which are a One-Hit Kill against infantry. The Imperium, on the other hand, have to deal with waves and waves of infantry which can overwhelm Neutron Accelerators through sheer force of numbers fairly easily. In addition, the Freedom Guard are guaranteed to throw at least one Shockwave at the Imperium, which usually happens very early in the mission, before you have the army built up enough to handle a defensive breach.
The second level of the Saladin campaign. Where to start? The fact that you are still restricted to the Castle Age, while your enemies can go all the way up to Imperial? The way that you start out with almost nothing, up against Reynalds' Bandits, which consist of a small army, and the Raiders and Pirates, both of whom are building their own? The knowledge that you have to protect two almost-useless allies who do nothing but scream at you every time the enemies look menacingly at one of their caravans? The way that there are almost no resources near your base, forcing you to herd sheep across the map just to get enough resources so your enemies don't wipe you out with one misplaced sneeze? The way that even if you somehow manage to destroy the first two enemies, the third is probably so well entrenched by this point that it'll take an hour and a half to defeat him?
Topped by Jihad!, the fifth mission. You're tasked with destroying two of the three Crusader city-states: Ascalon, Tiberias, and Tyre. Tyre is located across the sea to the northeast, Tiberias to the west, and Ascalon to the south. Yup, you're right in the middle, on the coast, perfectly exposed to attack from all sides. Tiberias and Tyre will waste no time throwing camels, heavy cavalry and rams (Tiberias) or galleons, cataphracts, and bombard cannons (Tyre). Meanwhile, Ascalon goes for the Wonder, meaning that you can't just wage a war of attrition, you have to go on a major offensive to gut Ascalon before the timer runs out. On the bright side, a Good Bad Bug sometimes means Ascalon starts with no AI, but even then it's still That One Level.
Then The Lion and the Demon tops Jihad!. You are required to build and defend a wonder in Acre, while holding off five different Crusader armies: Genoese (warships), the Franks (cavalry, gunpowder units, and rams), Knights Templar (rams, Teutonic Knights, and cavalry), Jerusalem (camels, onagers, and trebuchets), and Richard the Lionhearted (longbowmen, cavalry, and mixed siege weapons, including two unique trebuchets). You have some resources inside your walls, but they'll run out quickly (especially your very limited wood supply). There's no real way to bottleneck the attackers; they come from both north and west, almost constantly. This is widely considered the hardest scenario in the game, and for good reason.
How about the Lombard League? You get four enemy factions to fight; Padua, who starts out aggressively attacking; Venice, who rules the sea; Verona, who will eventually start attacking; and Henry the Lion, who starts as your ally but promptly betrays you, because having an ally might make this level, you know, possible. The objective is to build a Wonder inside one of the Italian city-states' walls, but good luck even getting a single unit inside the walls. You start off getting chased out of your base by (who else?) Padua, which will pretty much set the tone for the rest of the mission. You can relocate to one of two places, neither of which has much in the way of resources, and one of which is right on Padua's and Venice's doorstep.
Age of Empires II packs more of there. The Pope and Antipope scenario in Barbarossa's campaign seems fairly bearable at first - the only real enemies are across a river and their periodic coastal raids can be fought off with towers. However, the player has to cross the river to defeat the Milanese forces, who are not only equipped with an AI that's both extremely territorial and quick to repair damaged walls, but who also have the resources to keep churning out more troops for hours. And if you fail to destroy that harmless little town to the south, they'll supply the River Guards and Milan with additional resources, meaning you'll be dealing with transports full of Milanese soldiers steaming up the river.
There's also "The Siege of Paris", the second-last mission in the Joan of Arc campaign, an Escort Mission with NO HEALERS to support your forces. To make things worse, you will have to go through a heavily guarded city fill with longbowmen and halberdiers. You will also have to rescue the villagers in the city and AT LEAST SIX OUT OF TEN of them must reach the destination.
Into China, the third mission in the Genghis Khan campaign. You have to fight four Chinese tribes: Hsi Hsia, Tanguts, Jin, and Sung. Three of them are behind the Great Wall, and Jin is in an island stronghold in the western corner of the map. Since none of them are particularly aggressive, this would be an easy scenario, if Jin didn't try to go for a Wonder victory, and quickly. Since Jin gives you so little time before they start on the Wonder, you have to work like crazy to get to the Imperial Age and scramble a force that can punch through Jin's army of Chinese archers and heavy cavalry to reach the Wonder and raze it. On the bright side, once Jin's Wonder goes down, the rest of the level isn't particularly hard.
Pax Mongolia, the final level of the Genghis Khan campaign, pits you up against Hungary. It starts with you defending against the Hungarian knights crossing the Sajo River bridge while waiting on Subotai's reinforcements. A little while after Subotai arrives, the Hungarians blow up the bridge and start working on a Wonder, forcing you to cross the river in a different spot and invade their base. This wouldn't be too hard, if the game didn't give you a stupid amount of Saboteurs (ten at first, and another ten when Subotai arrives). With a population limit of 75, having twenty one-use units takes up a lot of valuable space, but you won't want to delete them because they're such powerful units (four times as powerful as Petards).
From Age of Empires, there's The Great Hunt. An infuriatingly long level, where you have no base and have to wander back and forth across a huge map to recover an artifact at the very far north. You start out with just a few axemen and have to dash madly past enemies you can't hope to defeat. Then you find some priests and have to convert practically every enemy you come to, just to build up some semblance of an offensive force. If you lose your priests, you're screwed (and converting catapults and elephant archers never gets any easier). Then, you get an annoyingly long water segment, loaded with ballistas and catapults shooting at your transports. If you lose your transports, guess what? Time to load a save. After you struggle through that nightmare, you have to ascend the final peak, with a timer, with hordes of catapults and late-game units trying to kill you. Only after you reach the top and find the artifact are you done with this horrible, horrible level.
Nineveh, Nineveh, Nineveh. You start out with a sizeable base, loads of resources, a small standing army...and no villagers. You have to go out and convert an enemy villager without provoking unnecessary wrath, then come back to your village and build up enough to upgrade to the Iron Age. You need to be in the iron age to assault Nineveh, which is building a wonder. After advancing, you have to fan out to find more resources whilst avoiding conflict, since you don't have time to fight off an attack. Meanwhile, you have to chip away at Nineveh's nigh-impenetrable defenses so you can conceivably land a force without getting massacred. Then, you have to invade Nineveh and smash through walls, towers, and a tremendous defensive force to destroy the wonder before time runs out. On the easier difficulty settings, this is a pushover because the AI is too stupid to assign multiple villagers to the wonder, but it's brutally Nintendo Hard on Medium and above.
The first mission in the Yamato campaign, 'The Assassins'. You have to kill the leader of a nearby enemy tribe, and are given three broad swordsmen, a cavalry, and an archer. This involves trekking all the way down a fairly large map along a road infested with lions and elephants, all of which are aggressive. The lions aren't too much of a threat, but if you stumble across an elephant, it can easily put a serious crimp in your assassination plans. Then, you work your way into the base, which is loaded with catapults, towers, long swordsmen, cavalry, and heavy infantry, all of which have been upgraded multiple times (and yes, your party is all at base stats). And the leader himself could easily fall into That One Boss territory on his own, as he's ridiculously powerful and has staggeringly high HP.
Possibly outclassed by 'Island Hopping', the second mission. You have to make your way through a bunch of islands to capture six artifacts. For starters, unless you proceed to the island southwest of yours immediately after the mission starts, an enemy will capture an artifact and trigger a timer. Then, you have to cross the map to find an enemy dock, which, if left unchecked, will fill the map with enemy war galleys. Failing to do either of those makes the mission almost impossible to complete. Did I mention the units you're given for this: two war galleys, a light transport, a heavy transport, two phalanxes, three composite bowmen, a catapult, and a ballista. That's it. Lose the heavy transport or that catapult and you can kiss victory goodbye.
The Yamato campaign can really fall under That One Campaign. The fourth level, Mountain Temple, is absolute murder on higher difficulties. Your objective is to destroy the Izumo mountain fortress. Sounds simple, but you start in the Stone Age, and there's two Kibi tribes you're competing against for resources. One of them is in the Stone Age like you, and the other is in the Bronze Age. You essentially have to speedrun the first half of the mission to wipe out the Kibi tribes before they devour the (very limited) resources on the map. After that hell, defeating the Izumo and building the temple is cake. You did remember to save the 200 or so wood it takes to build a temple, right?
Tucana from Creeper World. You start in the center of the map with creeper generators at all four corners, and there is no high ground large enough to move Odin City to. Of course, once you know how to beat it, you can hold the creeper off with only two mortars
The level is so infamous, when the player-created level pack was released, one of those levels went meta, resembling a computer playing the Tucana level, and spilling out of the (in-level) monitor.
Level 4-3 from the flash game Paladog. The levels that restrict the player's selection of units and spells had always been annoying, but this one takes the cake. Wave after wave of miner goblins, all of which will easily take out Paladog if even one of them gets close. Your main force consists of the vastlyOverrated And Underleveled Monkey the Pirate, who can barely scratch the goblins, even in groups. Your backups are Defensive Tortoise, who can't attack at all and gets overrun easily, and Penguin the Wizard, who is actually useful but is a Glass Cannon with the inexplicable tendency to get close to the goblins. The only spells you can hold are Fire (which is so close-range that by the time it would be useful, you're probably already dead), Ice (which only hits one enemy), and Turn Undead (which is a purely luck-based One-Hit Kill). About the only "strategy" that will work here is repeated abuse of Turn Undead, which means lots of prayer to the Random Number God.
Power games in Anno 2070 is the only three-star difficulty mission in the original game, for very good reason. The victory conditions are as follows:
Eliminate all 4 opponents. This includes a pirate who you can be sure has many more ships than you do, and will throw every single one at you from the start of the game if you don't constantly bribe him, an oil baron who will complain and outright attack you for not buying his hideously overpriced oil, A militaristic guy who manages to throw aircraft at you before you have any way of fighting it, and worst of all, Keto, with her flagship Anaconda and dozens of Sharks in tow.
Build both an Eco and Tycoon Monument. Monuments take literally two hours just to build the thing, and eat up power, income, and ecobalance like no tomorrow. Yes, even the Eco's Leisure Center.
Accumulate 1 million credits. not particularly hard, but very time consuming.
Settle 5000 Eco executives, 5000 Tycoon executives, and 2500 researchers. Again, not particularly hard, but very time consuming. By the way, the average time it takes for a good player to finish this level is over twenty four hours.
In Stronghold, the original one that is, there's mission 15 and 18, both of them dealing with taking out The Pig's castle. In both of them the castle is heavily fortified and defended, while you are given a rather small, non-replaceable quantity of troops. Winning this mission on anything above Easy difficulty is an exercise in patience and micromanagement, and even on Easy it's no walk in the park. Especially odd since the rest of the missions aren't too hard.