Demonic Spiders / Etrian Odyssey

As expected of a Nintendo Hard game series, there is bound to be at least one enemy per stratum that easily qualifies for any party traveling through for the first time, almost mandating the player to use a specific strategy to at least incapacitate them.

Multiple Games

• Across various games, there are monsters that are weak on their own, but if you don't kill them fast enough, they will summon or transform to F.O.E.-type monsters. A really nasty surprise if you don't expect it.
• Anything brandishing the poison ailment can make short work of parties in the early game as the ability to purge ailments and heal is not easily available, and the poison damage ticks for much faster than your healing methods can sustainably handle.
  • Venomflies deserve special mention here. In the first game, they showed up in the first stratum, with an attack that poisons one member of your party. Annoying, but manageable. In the second game, they showed up, still in the first stratum, with an attack that poisons your entire party. Then, in the third game, they showed up in a locked area of the first stratum, with an attack that poisons one member of your party - at a rate of 350 HP per turn of poison damage.
• Moth-type enemies have a nasty habit of spreading confusion to your party, rendering them uncontrollable and unable to dodge the Moth's other attacks. God help you if your healer's hit. Sometimes their non-ailment-inducing attacks hit hard and frequently enough that despite their low accuracy they will roll some good hits and take out a party member or two.
• The final floor of each stratum typically introduces one last enemy that's a fair bit more powerful and deadly than the rest as a random encounter.
• Any enemy who has Instant Death abilities, especially ones that hit multiple characters and come in packs. While some are balanced out by requiring a turn to use to give time to bind or use a sonic bomb to stop it, it's still easy to either mess up or have them paired with disabling enemies and quickly end a game.
• FOEs themselves would easily qualify if not for the fact that the player can easily plan their movements around them to avoid getting caught in battle. However, faster and more aggressive FOEs are harder to flee from, and would be listed accordingly.
• Several monsters can perform Combination Attacks, with one or more monsters "standing ready" and another monster performing the attack itself with the aid of its partners. If at least one of the participant monsters isn't killed before the attack takes place, or if you at least don't render the main user unable to use the skill (binds, Panic, etc.), expect to see at least two or three dead bodies in your party at the end of the turn or even a Game Over.
• If an enemy is weak to all elements, all physical attack types, or both, watch out. They will often have a skill that is designed to punish those who try to capitalize on those weaknesses, such as a way to inflict the Curse ailment or a Last Ditch Move that inflicts debilitating debuffs.
• Enemies with Powerful, but Inaccurate attacks should never be underestimated. They will often have a way to reduce or disable your evasion so that those attacks will hit and likely get multiple or all party members killed.

Etrian Odyssey and The Millennium Girl

• Petaloids. Their variants in Etrian Odyssey loved nothing more than to chain-sleep your entire party, thus allowing the other enemies to unleash a world of hurt on you. They appear in Legends of the Titan as well, although thankfully they'll only target one party member... unless you damage them.
• The Bloodant FOEs in Etrian Odyssey. The first time you encounter them on B12F, they don't hit very hard, but have a lot of HP. Plus, they have infinite regeneration and the ability to summon both Bloodants and Deathants. To top it off, with the battle against the Royalant, they'll continually spawn in a corner of the room and rush towards the battle. Enjoy a main dish of Royalant, a side order of Bloodants with a generous helping of Deathants with your-ass-getting-kicked sauce. Thankfully, they are much easier in the remake.
• Depth Dancers are one of the most irritating FOEs in The Millennium Girl. They move a whopping three squares per player step, and spawn randomly out of the walls, two at a time, intending to divebomb the party. The area where they are first encountered has a lot of open space for maneuvering around them... and plenty of damage tiles.
• Evilroots of The Millennium Girl commonly take a turn to charge up, which telegraphs their Last Cry skill. This is a very powerful party-wide instakill, which makes them priority targets or at least a reminder to carry Sonic Grenades around.
• Iron Crabs are found on the final floor of the bonus stratum in The Millennium Girl. They are docile, resistant to most forms of attack, but if you kill any of its allies it will turn aggressive and spam Scissor Wrath which can destroy even the strongest of parties. The game loves to pair them up with other deadly encounters like Red Corpuscles that with an instant death attack, but worst of all is when it gets paired with Evilroots, detailed above. You either kill the Evilroots and die to Scissor Wrath, or you kill the Iron Crabs (or fail to run away) and die to Last Cry... ^note Or die to the Depth Dancers that are swooping in on your position.

Etrian Odyssey II: Heroes of Lagaard and The Fafnir Knight

• A clear example from the second game are Gigaboars, since it's the stopping point for many in the second stratum, forcing a serious training and reequipping stop. Gigaboars don't have much HP, all in all -— 180 is a pretty manageable amount for a party at that level. But they hit like mack trucks. As in, if you're using a Ronin or Dark Hunter in the front lines? Oneshotted. And they have pretty decent speed too, so you can't use the most damaging attacks, since they're slow and you'll give them the first action that way.
• Mandrakes are a first stratum enemy in the first game, but in Heroes of Lagaard they don't appear until the fifth, towards the end of the main game. There's a very good reason for this. The Mandrake's only skill, Scream, attempts to inflict instant death on the entire party at a not-insignificant chance of 35%. Their best stats are conveniently Luck, making them more likely to inflict it, and Agility, letting them use it faster while also being more likely to dodge any attempts to frantically take it out. It at least only starts using it after taking damage, but that doesn't apply to blindsides, where it has a decent chance of opening with it and wiping the party before they can even act (and they can come in groups). The Fafnir Knight essentially converts them into weaker Evilroots from The Millenium Girl, with the instant death chance of the skill (now Last Cry) being much higher but being interruptable with Blast Grenades, and it says a lot that this version is still considerably fairer than the original.
• Another star in the ending of the second Stratum of Heroes of Lagaard is the Poseidon. A huge grass brachiosaur with about double the HP of almost everything else in the floor, which doesn't hit for all that much less than the areas F.O.E.s, and which has a move named Scurry that hits your whole party - yes, including the Squishy Wizards in your back row - for huge damage. Additionally, it seems fond of blindsiding you (which poses the question of how in blazes you get blindsided by a goddamned Brachiosaurus), which means he gets a free turn to maul you into paste. And he almost never appears alone, to add insult to injury. Good luck avoiding them, since all resource-gathering points have a random chance of giving you a battle instead of an item, and Poseidons are a common opponent in these events.
• The third floor of Ginnungagap in The Fafnir Knight, also That One Level, is crawling with Death Walls. These disguise themselves almost perfectly as walls, and don't react until the player stops facing the party directly at them. They also don't show up on the map while they are stationary, and they can quickly close the gap between them and the party. Get caught in a battle with one, and the Death Wall also prevents you from fleeing the battle (except on certain turns), forcing you to expend a Return Flute. The later rooms of the floor also involve you navigating around 2 to 3 of them at the same time.
• The Muckdile (Dinogator in The Fafnir Knight) exchanges its unremarkable FOE status in the previous game for infamy as an incredibly deadly random encounter in the final floor. Its Bite attack hits the whole party incredibly hard, and it's explicitly designed to counter some of the strongest Force skills in the game. It's immune to Stun, invalidating Riot Gun's utility, and attempting to fully bind it with Dominate causes it to Frolic and fully bind the entire party. Your only out is to bind its head and only its head.
  • In the remake, the Dinogator becomes a lot more dangerous. Although not explicitly designed to counter some of the player's most powerful tools, it's a lot more difficult to shut down. A head bind still stops its deadliest attacks, but this just causes it to Frolic nonstop, forcing you to devote a party member to keeping a Barrier up all the time. When it Turns Red, its Pout attack pretty much devastates the whole party.
• The Anomalocaris in the DLC 31st floor of The Fafnir Knight has an astounding sight range of 5, but more infuriatingly, it moves randomly, making it nigh-impossible to traverse the floor without engaging one in combat. It only has two attacks: one that drains TP and can completely bind its target, and another that does splash damage with a chance of instantly killing anyone hit. Between these attacks and a high HP count, it's difficult to defeat a single one of them without experiencing significant loss of TP. On top of that, if you leave the floor, all of them respawn — you have to beat most of the floor in one sitting since you cannot kill the Anomalocaris to grant safe passage on return trips. Mercifully, a shortcut exists connecting the entrance to the boss so that you can challenge it with a fresh party, but it's a long trip through the floor to reach it, and the last room with these critters contains five of them.

Etrian Odyssey III: The Drowned City

• An early show-stopper is the Largebill, found as early as the second floor in the daytime. It deals enough damage to one-shot even low-level tanks, and has enough health to be hard to kill. Players in this stage of the game are often advised to explore at night to avoid this threat.
• Forest Shrimp, found in the Waterfall Woods behind the Star and Moon Doors. They have a special attack that hits the entire party and has a high chance of causing paralysis. They're also tough as nails, being highly resistant to both physical damage and Ice damage (which is a problem if you're hunting Golden Idols). Even more irritatingly, they tend to show up with Demon Octopodes, which have an attack that binds your entire party's legs, making it that much harder to escape them.
• Big Snakeheads, found all throughout the Abyssal Shrine. Huge fish with a devastatingly strong volt attack that hits the entire front row. They don't have any real weaknesses, and they're tanky enough to soak a few turns of attacks.
• The Longicorn Beta, found on the final floor of the Bonus Dungeon, can summon multiple enemies at any time, and then throws them for high damage to the entire party. It's also highly durable, which makes it difficult to shut it down with statuses long enough or kill fast enough to stop it from throwing enemies.

Etrian Odyssey IV: Legends of the Titan

• The Hollows are meant to be a crash course in utilizing Bindings, and it shows. Even the weakest Hollow type has evasion that's through the roof, so unless you have a Sniper or Arcanist in your party, just hitting one can be a Luck-Based Mission!
  • The Bonus Dungeon takes this to the next level with the Hollow Magus, which has a skill that grants a significant power boost to another enemy in exchange for doing single digits' worth of damage to them. This becomes problematic when combined with Muskoids (see below), or when it awakens a sleeping Red Lion which will do massive damage to the party. Naturally, the instinctive response is to kill the Hollow Magus quickly, but like its predecessors it has incredible evasion unless its legs are bound...
• Muskoids in the Bonus Dungeon can petrify your entire party with one move. While they only target one member of your party until they take damage, they're occasionally paired with an enemy (a Hollow, no less) that can deal that one point of damage to get a power boost.

Etrian Odyssey V: Beyond the Myth

• Colossal Ropers in Beyond the Myth don't seem all that bad when they're first encountered on 3F, barring their leg bind and multi-hitting attack, but come 5F and they often come into battle with Rabid Acorns as reinforcements. It will then throw those Acorns at you for massive damage to the whole party, quite possibly a Total Party Kill, if you don't either bind the Roper's arms or kill at least one Acorn first.
• Coffin Demons in the final floor of the third stratum and Hex Steeds in the fifth behave similarly: they have a multi-hitting attack that can leave behind a mixture of ailments. Coffin Demons inflict poison, curse, or blind; Hex Steeds inflict paralysis, curse, and petrification. They also are tankier than every other random encounter in the stratum. Disable them quickly or your party will meet an untimely end.
• Shielded Phasmids in the secret areas of the Tutelary Forest have two types of Counter Attacks. The first causes it to retaliate for each of its allies that have been felled this battle, which can be easily accomplished if you're not careful with area-of-effect skills. The second causes it to counter physical damage. Only three classes out of ten can inflict pure elemental damage without any physical component. Both attacks hurt really hard and can cause unsuspecting parties to wipe out.
• Fire Squirrels, found in secret areas of the Jagged Reach, serve as a very nasty Call-Back to a series of Schmuck Bait events in Heroes of Lagaard and its remake. Not only does it hit fairly hard, if it's the last one standing (and it almost always will be, since it's grouped with much weaker enemies) it uses Burning Thread, which destroys an Ariadne Thread. Floor Jump does not exist in this game, so they can easily strand you if you don't kill them first.
• Dreamrays in the Bonus Dungeon love to spam a party-wide ice attack that has a good chance of putting any hit targets to sleep. They are also immune to the bind which would otherwise shut down this attack, and are resistant to most ailments, making them difficult to shut down before they act.
• Asura Cicadas spread a severe defense debuff on your party, leaving them more vulnerable to other more deadly enemies. When they die, however, they unleash a party-wide ice attack which is likely to cause a Total Party Kill if the defense debuff is active.

Etrian Odyssey Nexus

• The Ananas series of enemies, which are found in Shrines starting in the second one. They are vulnerable to all physical attack types...which is classic Schmuck Bait, because upon dying, they release a final attack that inflicts disabling status ailments or binds, most certainly a death sentence if you make the mistake of not saving those enemies for last. While you can simply avoid targeting them or using skills that target random or all enemies, they become problematic once again if you have a character with a Counter-Attack passive.
• Cocky Capacitors are found in the Buried Castle. On their own, they're pathetic, incapable of any form of offense and actually opting to run away from the party on the field. When two meet, though, they fuse and become a lot more powerful and aggressive, moving fast enough to chase down the party. Not helping matters is the cramped environment making escape difficult. Defeating it is not easy either — when it falls to low health, it splits into four small Capacitors, and if they are not killed quickly they fuse into a large Capacitor at full health, starting the fight over with your resources depleted.
• Muskoids return in the final dungeon, and this time they can petrify the entire party without having taken any damage! Although the game no longer gives you an instant game-over if the whole party is petrified, on top of the ailment wearing off mid-battle, a petrified party is still ripe pickings for the Muskoids' companions which shower you in elemental attacks.
• Archpixies in the Illusory Woods have two skills: One instantly kills a party member, the other can cause triple-bind on a row. They're also immune to all ailments barring instant death, and take only Scratch Damage from pure elemental attacks. If a group of four show up, it's best you run immediately. Your one hope is a head bind, but good luck if your binder gets killed or bound.
• Hexgourds, the FOE of the Illusory Woods, are not just aggressive FOEs, but are also capable of following you through walls. The moment you've earned its attention, there is no escape except by returning to town. If you do try to engage in battle, you quickly realize that all your attacks do Scratch Damage to it, making battle a rather futile endeavor. There's a reason the game warns you to keep an Ariadne Thread on hand.
• The Dinogator makes its return as the strongest FOE in the game, and this time it holds nothing back — the Pout skill normally reserved for when it Turns Red is now used on the very first turn, hitting for over 2000 damage per hit. There are two ways to go about this fight: either force it to sleep to avoid the turn 1 Pout, or capitalize on Pout to kill it with Curse backlash damage. Fortunately, it's very difficult to encounter the Dinogator by accident, making it more of an unconventional superboss.