That One Boss/Shoot'Em Up/Touhou

""Still, is it too easy? Well, I'm about at my limit...""

- ZUN on Perfect Cherry Blossom, the only Touhou game to have a phantasm stage - a level above the usual extra stage.

""I can see it... My mind-reading third eye will show me your very heart! An image of battle... I think I can use this to make you suffer. [...] Now, this is where it really begins! Go and sleep with a trauma so frightful it will wake you up!""

- Satori Komeiji, Subterranean Animism, stage 4.

As anyone who has played Touhou will tell you, the boss fights are what these games are all about. There are well over 100 boss characters in the series, and you encounter most of them only once, so it's natural that some make a more lasting impression than others. In Fanon, they might be famous for their colorful personalities, ridiculous cuteness levels, or Memetic Sex God/Molester status, but for those who have actually played the games, they will always be fondly remembered for a different reason.

She may not be the hardest boss in her game. She may not test your skills to the same extent as the Final Boss, or your patience and determination like the Extra Boss. Beating her may not even earn you much bragging rights. But she will make your life a living Bullet Hell nonetheless. She is the reason many people have never seen the good endings. She has left countless broken hearts and keyboards in her wake. Even hardened shmup players have been known to reach for the bomb key at first sight of her. She is That One Loli!

Which boss exactly qualifies for the title varies from player to player and depends somewhat on your choice of character and shot type, but generally every game has at least one boss that will make you go pichuun again and again long after you've figured her patterns out. You can always count on them to mess up your until-that-point perfect game. They are mostly found around stage 4 or 5.

NOTE: As always, the Final Boss and "Wake-Up Call" Boss cannot be That One Boss without being overly hard by their standards. Please do not nonchalantly add them as examples. Also, to avoid cheapening the title, don't list bosses that are only an example on Lunatic difficulty or because of that one spell card you can cancel out with a single bomb. Remember Bonus Boss is completely banned NO MATTER WHAT.

The Story of Eastern Wonderland

 * Marisa Kirisame in her very first appearance as the stage 4 boss. She pulls out four multicolored orbs that all shoot danmaku in addition to the bullets coming from herself. Furthermore, she can cause them to spin around outward in a fashion that is very hard to anticipate and dodge. She becomes much easier when the orbs are destroyed... until she simply recreates them. She is also aggravating due to her tendency to fly down to the bottom of the screen, much closer to the player than most other Touhou bosses, which makes dodging her attacks harder or impossible.

Lotus Land Story

 * Elly, mainly because of her scythe. She throws it at you in the middle of her patterns, which already require a fair bit of precision, it's slightly homing, and it blocks your shots. If you're playing a shot type that can't get around that it'll be a very long fight.
 * When playing as Marisa, there's Reimu in stage 4. She is a bitch to defeat, since she has several attacks that can either spawn very fast spreads of bullets, yin-yangs which are large and bounce off the sides of the play field in erratic fashions, or wall in the player and force them to dodge fast aimed bullets.

Mystic Square

 * Mai and Yuki, the stage 4 bosses, are pretty frustrating due to the sheer amount of bullet spam coming from two different directions, as well as Mai's laser and small bullets. And that's when you fight them together. Depending upon who received the most damage, you fight the survivor, and it gets harder. Mai uses bouncing ice daggers and egg bullets that break into more bullets when your shots hit them. Yuki, on the other hand, just spams slow and fast patterns, but also uses fireballs that block shots.
 * Yumeko, the penultimate boss of the game, is known for being a nightmare to defeat even for experts. While the difficulty of the game is gradually but steadily raised until her battle, she herself is a nasty Difficulty Spike. Half of her patterns require very fast reactions - she just throws some knives and bullets. But later, you have to deal with VERY tightly aimed streams of bullets/lasers AND knives she randomly throws everywhere. If you survive that, prepare for some very fast bullet walls followed by more knives... from both sides of the screen! Her final attack, which "only" requires dodging diagonally thrown daggers, seems easy by comparison. And since all of her patterns are speedy and need very good reflexes rather than good bullet trajectory reading, you will usually have to bomb in advance to avoid losing even more resources. Just watch this video...

Embodiment of Scarlet Devil

 * Patchouli Knowledge, the stage 4 boss, is the novice Touhou player's greatest obstacle to completing the game without continues. Her spell card patterns are very random, and she tends to move around a lot, making her a challenge to hit. If you chose to play as Reimu B, who is otherwise the most balanced character, you're stuck with torturous spell cards like "Bury in Lake" and "Water Elf".
 * If you got past Patchouli, there is no chance to rest, as Sakuya comes very soon after. Her midboss pattern is specifically designed to box the player in. It does not help that she is the first user of that annoying dagger danmaku with a hitbox which seems larger than the actual sprite. Her stage 5 midboss spell card "Misdirection" can drop your lives quickly if you don't know exactly how to deal with it. The name of the card is the key to success.

Perfect Cherry Blossom

 * The Prismriver Sisters, come at the end of stage 4, which is already arguably the hardest non-extra level in Touhou. Yet again, the cards they use on you depend on your character choice, as well as which one of them you focus your shots on. Their nameless non-spell patterns fill the screen with a truly ridiculous number of bullets on higher difficulties. These are the same every time, so it's possible to find a safe spot and/or memorize them, but if you get hit even once while dodging them, Dynamic Difficulty kicks in and changes up the pattern, making memorization completely impractical.
 * The main downside to playing as Sakuya is you're forced to fight Merlin at the start of the Prismriver Sisters battle.
 * Once you get past the Prismriver Sisters, there is ANOTHER difficulty spike awaiting you - Youmu Konpaku. She keeps you company all the way to the final boss as miniboss of stage 5 & 6, but it's her boss battle that's evil. Her favorite method of attack is to throw dense AND rather fast (or just extremely dense) clusterfucks layers of bullets, while giving you about 3 seconds of bullet time to position yourself out of harm's way. In theory. You need a lot of practice to get used to this to the point you won't be reaching for the bomb key during every slowdown. Youmu singlehandely proves that reading-based spells can be just as hard as reaction-based ones. Demonstration.
 * Ran Yakumo. No, I don't mean her Extra Boss battle, which is actually pretty easy by Touhou Extra Boss standards. I mean her mid-boss battle in Phantasm stage. While the Phantasm boss, Yukari Yakumo, is agreed to not be the hardest bonus boss in the series compared to the likes of Flandre or Koishi, Phantasm midboss Ran is the only single enemy in any games that has the bomb immunity for all of her spell cards and is not a bonus boss (at the moment). Her spell cards are: a difficult combo of a wide stream of pseudo-aimed bullets (they slightly curve to hit you so you can't just slightly tap) and dense layers of more bullets to dodge, followed by a hellish spell where she spins around the screen alternating between circular and S letter motion and throws bullets in every direction, which you HAVE to dodge unless you shotgun her with a powerful shot type and use bomb invincibility smartly. If your shot type is weak, she can eat your resources in a flash.

Imperishable Night

 * Both level 4 bosses, that is, either Reimu or Marisa.
 * Reimu uses heavily patterned, but extremely dense clusters of spell tags, fires off powerful homing shots, and boxes you into small areas with her attacks. In the second part of the fight, she's usually zipping all over the screen, making herself nigh-impossible to hit. Not so hard as she was in LLS, but really hard anyways.
 * On the other hand, Marisa's attacks are much less structured. She prefers to shower the entire screen with her deceptively large looking star bullets. Her first spell card generates spirals of huge stars, showers the field with tiny stars coming in from the sides, and adds periodic shotgun-style blasts from her familiars. Then you get to the second half of the fight...
 * Keine Kamishirasawa, when you first encounter her in stage 3, is an excellent example of a "Wake-Up Call" Boss, her patterns getting much easier with practice. But when you encounter her as the extra stage midboss... well, let's just say she lives up to that one meme she's best known for. Her attacks shower the entire screen with a truly absurd amount of bullets, including very fast bullets that cross over each other, bullets that track the player at the same ridiculous speed, very slow bullets that will almost certainly throw you off when used alongside the former, and bullets that head toward Keine and will almost invariably trap you. For added confusion, the patterns change anytime you press or release the focus button. It probably says something when most of Mokou's spell cards are much easier to deal with, Fujiyama Volcano notwithstanding.

Phantasmagoria of Flower View

 * Aya Shameimaru. She's fast as hell, and due to the AI in that game, she will be a hard opponent.

Shoot the Bullet

 * "Seamless Ceiling of Kinkaku-ji". The nightmarish new impossible request of Kaguya Houraisan is impossible for many a player indeed. It starts as a seemingly doable pattern of falling almost half-screen bullet walls and a spinning layer of bullets thrown around the stage to make it harder to approach her. Said layer gets thicker after each successful photo and reaches a ridiculous level for the last one - it's one of the scenes many people simply don't have the patience to clear.

Mountain of Faith

 * Aya, from the past main game, returns as the stage 4 boss. The luck factor is vital for that fight, especially the survival card almost at the end.

Subterranean Animism

 * Parsee Mizuhashi, despite only being a stage 2 boss, has become rather infamous for surprising players with how difficult her spell cards are. "Green-Eyed Monster" is particularly brutal, as she uses it when she encounters the players as a midboss, and it's homing, a quality typically reserved for the bosses of stages 5 and 6 in a Touhou game. Another gimmick not normally seen until endbosses is the Doppelganger Attack, which Parsee also uses (her "Tongue-Cut Sparrow" spell cards).
 * Yuugi Hoshiguma in the stage 3 usually trips up players due to the sheer brute force of her attacks. Her final spellcard, "Knockout In Three Steps", is particularly brutal. She's often cited as a reason to not assume a character's strength from their placement in the game. One such argument spawned the NOT EVEN DROP meme.
 * Satori Komeiji can be this, depending on who your option is, as she uses that character's spell cards. Heaven help you if you're playing as ReimuC and aren't prepared for "Tengu Macro Burst" to come hurtling at you. If you are playing as ReimuA, then have fun with Yukari's "Border of Wave and Particle", which was part of the extra stage of Shoot The Bullet. And then there's MarisaB (Patchouli). Her torturous water cards are the ones that Satori chooses, even if one of them ("Princess Undine"), isn't one of the spellcards that Marisa remembers from her. Well, those and one of Patchouli's cards from the extra stage of EoSD, the "Philosopher's Stone".
 * Orin is almost universally declared the worst of them all. For beginners, there's the non-spellcards from the cat form in the stage 5 battle. Not so hard as it seems, but you can get clipped easily. Then there's the infamous "Cat's Walk" / "Vengeful Cat Spirit's Erratic Step". If you don't read the card's pattern correctly, well... And then, there's the actual boss battle. The zombie fairies are the least of your worries. Her third spell card, "Needle Mountain of a Former Hell" / "The Needles of Yore and the Vengeful Spirits in Pain", is really hard to read, due to the spinning ghost wheels, which are almost but not entirely unlike any other attack found in the series, and the bombs are useless against the wheels, so have fun.
 * Sanae Kochiya. While she isn't that hard in Mountain of Faith, she returns as an extra stage mid-boss in this game to torment players who want to just see the extra boss. Her attacks are a static laser + bullet combo that you HAVE to know a semi-safespot in order to dodge it (bottom, over the "Enemy" sign.), and even then you have to make precise movements to avoid ramming into the lasers, followed by a peculiar and difficult reading-based card followed by utterly insane aimed stream + random bullets combo that lasts WAY too long. Amusingly, many people simply use a safespot to survive this card (it needs some very careful positioning and some practice or you'll lose your resources instead of saving them), or simply use a bug to get rid of Sanae entirely (unfortunately, you don't get life parts from her if you do so, and you can only do it with 2 options out of 6, Reimu A excluded.)
 * With this many That One Boss characters, some fans like to joke that Subterranean Animism is That One Game.

Undefined Fantastic Object

 * Ichirin, together with Unzan, in stage 3. Be prepared to deal with Unzan's fists while you try to dodge the other bullets and lasers.
 * CAPTAIN MINAMITSU MOTHERFUCKING MURASA, the stage 4 boss. Giant anchors, literal raindrop dodging, and a homing survival card, all at stage four. If she doesn't mess you up, then...
 * Shou Toramaru, the stage 5 boss, definitely will do it. Many people have great difficulty with her, especially with the curvy lasers. Her most notable spell card is "Most Valuable Vajra" / "Vajra of Perfect Buddhism". The spinning homing lasers plus the other bullets make this card insanely difficult. It gives you so little opportunity to damage her, that with certain shot types it basically becomes a survival card too.

Double Spoiler

 * Satori's "Vague Recollection of Kinkaku-ji". See that example in Shoot the Bullet section? Satori, being the troll she is, makes the players relive the horror again. This time, you only have to take 3 pictures... because the spell starts in the hardest mode. And the bullet pattern before the last photo is even harder than the original spellcard's hardest phase.
 * Iku Nagae, the stage 10 boss. Though she originated in the fighting game Scarlet Weather Rhapsody, and was not even the final boss in it, practically all her spell cards are a nightmare. Just try and capturing "Pearl of the Five-Clawed Dragon" with Hatate, who makes every other card in the game easier. And then you get to "Dragon Palace Messenger's Swimming Shot", which feels downright broken. No strategy or trick works against its combination of completely random curvy lasers mixed with completely random large orbs with big hitboxes. This is the last scene most people clear, usually by pure luck, if at all.

Ten Desires

 * Yoshika Miyako, the resident jiang-shi of Stage 3, has the annoying gimmick of absorbing spirits that emanate from her body to restore health. If you don't get close to her to intercept the spirits, she can easily deny a spellcard capture, even if you are wailing at her health the entire time. In addition, she fires large, spread out waves of kunai danmaku that can be very difficult to deal with, especially with a forward shooting character. And that's not even getting to the problems Youmu has...
 * Seiga Kaku by herself is weak, but she decides to bring in a body guard which just so happens to be the previous That One Boss, which makes her incredibly hard to hit. Fortunately you can take Yoshika out. Unfortunately, she'll come back to life after a few seconds.
 * The extra stage midboss . Unlike most other extra stage midbosses, there really aren't any tricks to minimize any of her spellcards; you have to rely on pure dodging skill. Her first spellcard features bullets that can change trajectory with no warning whatsoever and all look exactly the same, and her third spellcard has large bullets aimed at you from every direction as well as large bullets moving towards the boss that can easily trap you. Both of these are quite nasty, but they pale in comparison to the second spellcard, which is basically the final spell card from the extra boss fight in UFO, only made harder. So what is arguably the single hardest spellcard in Ten Desires' Extra stage...belongs to the midboss. Yeah, good luck with that.

Fangames

 * Megumi Yaobi, the stage 5 boss from the fangame Marine Benefit. She has the gimmick of flooding the screen while giving the player a bubble to stay in; if your hitbox leaves the bubble for even a millisecond, you die. On top of that, she moves constantly all around the screen, making it very hard to actually deal damage to her. For her last nonspell, she even circles the screen while spraying bullets everywhere. And when you think it's over, she pulls out her survival card, The Eight Million Laughing Gods, which is That One Attack to end them all. It's not unheard of to lose two or three lives to this one card.