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(Zoe, Jayne and Wash enter and see Mal grappling with his torturer. Jayne raises his gun) Zoe: Jayne! This is something the Captain has to do for himself. Mal: N-No, it's not! Zoe: Oh. (Zoe, Jayne and Wash riddle the sap with bullets)
Happens a few times in Bleach with Ichigo; he sets out repeatedly to fight alone, and most of the final battles are just him and the Big Bad. But his friends and The Obi-Wan Urahara refuse to actually let him set off for battle alone.
However, Ukitake does give Rukia a speech which amounts to this concept and leads to the death of Kaien
Subverted in some manga chapters where Ichigo was fighting Aizen and was joined by every single Shinigami or Vizard who could still raise a sword.
Rukia says it during Ichigo's fight with Grand Fisher(the hollow who killed his mother), having a flashback to the aforementioned speech by Ukitake.
Averted in the Fullbringer arc where Ichigo and Uryu team up to fight Ginjo. Unfortunately, Ishida can't do anything to him, but Ichigo didn't seem to have a problem with accepting his help.
Justified in the Bount filler arc. Byakuya and Yoruichi are present at Ichigo's last fight with Jin Kariya, but they hang back so they can block his attempts to detonate the Jokai Crests, which would destroy Soul Society.
This occurs in almost every major battle (in the end anyway) in the Dragon Ball franchise.
Gohan: Sh-shouldn't we help him? Piccolo: Can't you see he has to fight this battle on his own? He's fighting for the honor of his fallen comrade... Tenshinhan: Help me, for the love of God, HELP ME! Piccolo: ...like a hero.
Samurai Deeper Kyo makes frequent use of this trope, to the point where the Shiseiten, a group of fighters, has a rule where no member may intervene in another member's fights. (Almost no one else in the series helps another in a one-on-one fight, but, yeah.) This leads to a scene where Akira, the youngest member, has to stand by and watch as the man who raised him in the group, Bontenmaru, is toyed with by Tokito.
In Naruto, Sasuke turns down Team Taka's offer to assist him in fighting Itachi.
Sasuke: Don't even think about it, Karin. This is my revenge.
During the Immortals arc, Naruto insists that he should take on Kakuzu by himself, as he considers it a "dangerous bridge" that he needs to cross alone or else he would be Just a Kid forever.
Naruto acts the same way on his fight with Pain.
In Yu-Gi-Oh!, Mai insists that Jounouchi duel Dinosaur Ryuuzaki without any advice. Jounouchi is willing to do this in order to prove that he can win on his own, but Honda opposes it.
In the manga, Jonouchi himself tells Yugi this on his own without Mai ever even bringing it up.
Inverted in Mahou Sensei Negima!, where Negi initially tried to do everything himself, only to get bailed out by his stronger allies. Although his Training from Hell is getting him the point where this won't be the case any more.
Negi's whole motivation for training like that is so that he can do everything himself. Although in this case it's not so much for personal validation as that he doesn't want his True Companions to be in danger because they had to bail him out.
Played straight during Wilhelm's attack and Chao's invasion, where Evangeline stopped Kaede and Konoemon respectively from interfering because she considered both events "training" for Negi.
Very poignantly done in an episode of Inuyasha, Jinenji, Kind Yet Sad.
In Digimon Tamers, Rika wanted Renamon to assist Impmon in a fight. She refused, because Impmon wanted to do the fight alone.
In Soul Eater Sid sees the fight between Black Star and Mifune as this when Nygus wants to intervene. Possibly justified in that Sid recognised the fight could determine whether Black Star risks ending up like his father or could find his own 'path' (it does, and he does), but also moral dissonance considering the honour over reason mentality.
In Sonic the Hedgehog: The Movie, Sonic initially insists that he has to defeat Metal Sonic alone. Though Sonic's apparently smart enough to know when he needs help—when Tails intervenes at a later point, Sonic thanks him.
Happens sometimes in Kenichi: The Mightiest Disciple, usually when a challenge fight is being fought, or when somebody desperately wants a rematch. Also interesting is the dynamics of helping and refusing help between Kenichi and Miu: Kenichi is very chivalrous and wants to become strong enough to protect Miu, and sometimes even tries to interfere when she is about to start fighting, but Miu is more skillful fighter than Kenichi (always beating him in sparring matches), and she is very willing to protect him when she thinks he is in danger. Kenichi, of course, often refuses such help from Miu, because "this is something a man must do himself".
In an earlier episode of Detective Conan, the murder happens at a reunion of Kogoro's old high school judo club, meaning both the victim and the murderer are friends of his. Conan pulls Kogoro aside and is about to tranquilize him as usual when he sees Kogoro's anger about the situation and his resolve to crack the case for his slain friend. Hearing this, Conan puts away his tranq gun and decides that Kogoro needs to solve this one himself, though he does still provide little hints.
Appropriately, as Trigun is a Stranded-On-A-Desert-Planet-For-Three-Generations Western, Vash the Stampede has been known to invoke this trope. For himself, he keeps trying to do things alone and keep everyone else out of the line of fire. More darkly, in the manga Vash is standing there watching for the horrible, gory, agonizing marathon battle that ends in Wolfwood dying of overusing his regeneration formula. He does nothing. At all. Despite the incredibly high stakes and the fact that that's his best friend and the fight is clearly completely unfair as well as heartbreaking on several levels. This is apparently his rationale, although it's hard to be sure why Vashdoes anything.
This also serves as a strong distinction between the anime and manga iterations of our hero. The anime is Lighter and Fluffier enough that its Vash could never do that. Ever. (His neglect in the corresponding incident consists of being too busy beating himself up for not being able to stop a random enemy with no lines from Eating His Gun to notice the blood trail. Not that noticing would have helped. Probably.)
Caine the Longshot is, interestingly, apparently the replacement for Livio and Razlo, whose character development is what took those two dozen or so bloody chapters. They gave him a sniper rifle and a mask, and he kills himself when Vash breaks the former. That is it. His whole character. Either really dutiful or really in love with his weapon.
A lesser example from the anime instead is the guys who take the card-playing rich girl hostage in the saloon in episode 3(?), when beard guy eventually gets his shot at his nemesis, Shinigami Bostalk, who murdered his parents and stole their life's work, and who his hostage knows only as Daddy. Vash respects the avenger's right to the showdown, although he cries with relief when he doesn't shoot to kill.
In Berserk Casca as a child was being sexually assaulted by a nobleman, when Griffith rode up to save the day. However, instead of saving her himself, he threw her a weapon and told her to defend herself, then just watched to see what the outcome would be.
Perhaps one of the only rules of engagement One Piece's Luffy actually pays attention to. Best highlighted early when Zoro fights with Mihawk. Zoro is losing handily, but Luffy refuses to intervene on Zoro's behalf or let anyone else do so. However, it's clear Luffy wants to jump in, but holds back from attacking Mihawk until the fight is actually over.
In Girls und Panzer, toward the end of the Little Army prequel manga, Miho is struggling to decide on a reason to pilot tanks, around the same time her friend Emi is set to return to Germany. Emi tells her and Miho's mutual friends Chihiro and Hitomi that even though Miho is burdened with doubt because of her family's philosophy it would be bad if Miho forgot that they became friends over tanks. Emi tells them not to tell her what they talked about, because Miho is "an idiot" who is unable to understand her sister Maho's advice to find her own reason for doing tanks.
Emi: It won’t be good for her if she can’t be liberated from the Nishizumi style. For her to continue with Panzer would be meaningless if she cannot change herself.
In Bakuman。, after learning that two of his friends and rival mangakas are threatened with cancellation, Nizuma gets uncharacteristically serious, saying that there's nothing he can do to help them, and that only those who have what it takes can succeed. They ultimately do get cancelled, and Nizuma is shown to be fairly unhappy about that result.
Nizuma: I... can't do anything for them. Only those who have what it takes can stay in this business.
Tuxedo Mask invokes this in Sailor Moon S when Haruka and Michiru challenge Usagi over her actions with Hotaru. The other girls aren't happy over the two against one fight and are ready to jump in before he stops them.
In Cardcaptor Sakura, Syaoran wants to help Sakura during The Final Judgement against Yue, but Kero stops him by saying that if anyone helps her she'll automatically lose.
This appears in the Munchkin cardgame by Steve Jackson Games in the form of the "Gazebo" card — which bears the text, "None can help you fight the gazebo. You must fight the gazebo alone".
This is an incredibly common trope in comic books. The comic's titular star will be facing an incredible threat, and it is only logical that other heroes would show up to help. However, that would remove the limelight from the star and make every comic into a "team" book. Therefore, writers are constantly having to invent reasons why a heroes' buddies are not available or cannot participate in the fight. The really lazy ones use this excuse. Notable incidents follow:
Carried to a ludicrous extreme in Green Lantern: Rebirth. Five Green Lanterns have to battle Parallax, the incredibly powerful, world-destroying embodiment of Fear. The entire Justice League shows up, and is involved in the story, but they let the Green Lanterns fight because they must do this alone.
One Superman arc involved a battle with the magical powerhouse Atlas. When Superman is on the verge of defeat, Supergirl offers to help. Superman insists he must do battle alone, and asks Supergirl to avenge him if he fails. However, Superman has no problem when Krypto the Superdog shows up and saves the day just a few moments later.
Done in the climax of The AvengersStory ArcKang Dynasty: Cap agrees to a one-on-one fight with Kang to conclude the war, with Thor preventing their teammates from interfering, citing this trope. The heroes protest vehemently, claiming that a monster like Kang deserves a group beatdown and not an honorable fight. Thor replies that its one-on-one not out of respect, but because Cap alone would be sufficient.
In Scion, Ethan and Bron have a big showdown on the Sanctuary island. The Heron royals and Ashleigh are about to help Ethan, but Skink insists Ethan must face Bron alone.
During the Silver Age while battling an energy creature that could possess and animated matter the entire Justice League of America sits back in their cave lair while Superman watches and reports back to everyone what's going on, but otherwise they do nothing because 'well he hasn't used his signal device to call us in yet so we'll just sit and watch'.
Subverted in BlackestNight, wherein Guy Gardner is forced to face his personal demons, or else he dies. Kyle Rayner tries to help him, but Mogo outright tells him this trope. Kyle's response?
In the second story-arc of My Little Pony: Friendship Is Magic (IDW) Princess Luna insists that she alone go rescue Rarity, being intimately familiar with the Nightmare Dreamscape. Twilight and Celestia quickly convince her that the other Spirits of Harmony (and Spike) can help, as well.
When Michonne in The Walking Dead leaves the party to confront The Governor she almost uses these exact words.
Rosario Vampire: Brightest Darkness Act III: In chapter 24, during Yukari's Designated Girl Fight with Apoch and Astreal over Ahakon, Tsukune, in accordance with Yukari's wishes, opts to just let Yukari finish the fight herself, partly because his Holy Lock might crack even more if he does, and partly because if they don't settle things on their own, then it'll just happen again. This decision has near-fatal consequences when Ahakon disrupts a Beam-O-War between the three and is seemingly killed in the blast, after which Tsukune confesses that he never thought that Apoch and Astreal would go that far; fortunately, it transpires that Luna intervened and saved Ahakon just in time.
Played straight in The Final Clash where Akuma demands a one-on-one fight with Ryu to the death. Snake and the others understand, but Alien Hominid goes after Akuma anyways. Yoda then Force Pulls him back to the party of heroes and says "This is something that he must do himself.".
The second Naruto event is subverted in The Sealed Kunai. Jiraiya states this regarding Naruto's fight with Kakuzu. Naruto wins, but promptly collapses afterward in utter exhaustion, causing Jiraiya to yell at him on ruining his "cool mentor" speech. Naruto makes a mental note to kill Jiraiya at some point in the future.
Also subverted in that Jiraiya claimed Naruto needed to do this so he could prove to himself he could defeat Akatsuki. Considering he already had Hidan's still-screaming head sealed away, it wasn't as critical as he might have thought.
In Christian Humber Reloaded, Vash stands aside as his Toa allies fight the Visoraks until they start losing, at which point he steps in and helps them by adding his energy to their combined attack, helping to finish off the enemy.
In Perfection Is Overrated, the Advisor points out that he only provides help to the Himes when doing so is necessary to ensure a fair contest against the SUEs, since it is ultimately up to them to prove themselves superior. He's also the one responsible for the creation of the SUEs, and gives them the same treatment.
Transformers: The Movie played on this, with Hot Rod and Kup arriving in time to help Optimus Prime against Megatron, but Hot Rod's attempt to help actually made the situation worse.
Also seen at the tail end of Transformers Cybertron, where even with Prime insisting that the war was over (which it was), Galvatron insists on a final showdown with him, and Prime agrees to face him one-on-one. (He still winds up cheating a bit with some help from the late Vector Prime.)
Shockwave: The secondary heroes stand around and watch, calling on their burning spirit and heart of justice to empower the main hero from afar.
Optimus Prime: Mmmm...no, we won't be doing that.
Subverted in The Incredibles. Mr. Incredible insists on facing the Omnidroid alone, despite having three family members with superpowers ready and willing to assist. Elastigirl (his wife) is upset, thinking that he just wants to show off, or prove that he's still a powerful superhero. But the truth is that he wants to go alone because he can't bear to risk his family's lives, having recently thought he'd lost them when their plane was shot down.
Films — Live-Action
In the movie Dungeons & Dragons, where Ridley enters a cave to search for the MacGuffin and the female Elf character prevents others from going too, because "he must do this himself". An explanation is attempted with a magical force field that keeps the others from entering.
Highlander has an explicit rule that battles between Immortals must be one-on-one.
Except this is more due to tradition than something they absolutely have to do. Kell in Highlander: Endgame has become so powerful partly by assembling a team of immortals, which he uses to kill others. Then he kills most of them at dinner. Hey, 'There can be only one'.
Played sort-of straight and simultaneously spoofed in Hot Fuzz. The hero is in a gas station, struggling with the decision of whether or not to return to try and save the village. After some inspiration thanks to seeing Point Break and Bad Boys 2 on DVD, he stops at the counter to purchase some badass sunglasses. The extremely wired-looking attendant asks "Is there anything else I can do for you, sir?" The response, of course, in a deep, gravelly, action-hero voice is, "No... This is something I have to do myself."
Subverted in Indiana Jones and the Kingdom of the Crystal Skull, where Indy explains everybody he was told by the skull to return it to its kingdom, a task he's got to do alone. On the next scene, however, we get to see all four sidekicks following him into the cave.
In the Forgotten Realms novel Azure Bonds Alias tells her party "Back away. He's mine!". Elminster not only refrained from intervention but prevented Nameless Bard from shooting heavily enchanted bolt into Phalse's eye, and arguing that "She must finish this battle alone"... "So she can prove to herself, and to thee, that she is her own master."
It's worth noting that in the hands of Grubb and Novak, Elminster is more often used to prevent a Deus ex Machina. Less the case in Ed Greenwood's...
In Dune Paul Atreides is dueling Feyd-Rautha and realizes that he could use a code word that was placed into Feyd-Rautha's subconscious to stun him for a second, thus saving his life. Paul refuses to do so because This Is Something He's Got To Do Himself. Interestingly, when he says, "No, I won't do it" (meaning use the code word), that is enough to distract Feyd-Rautha so Paul can win.
Victor: There are some things a man must do by himself.
Ginger: What, like... go to the toilet?
In Bernard Cornwell's Saxon Stories, Ragnar the Younger has one of these against his old enemy Kjartan. This trope name is actually invoked by one of the characters when the main character wants to intervene, saying basically, "Its his fight, and he will never forgive you if you interfere."
In The Lion, the Witch, and the Wardrobe (book and movie both), Aslan will not allow the Narnian army to interfere with Peter's fight against one of the White Witch's wolves, saying "Let the prince win his spurs." (book)/"This is Peter's battle." (movie) After Peter emerges victorious all on his own, Aslan dubs him "Sir Peter Wolf's-Bane, Knight of Narnia."
Lampshaded in End Game by Peter David (the finale of the pilot of the Star Trek New Frontier novel series), in which The Captain fights a desperate one-on-one battle on a planet which is blasting itself apart under him. He wins, only to find himself stranded amidst lava flows. His second in command then arrives in anti-gravity boots and rescues him, explaining that she knew he would insist on fighting alone, but now it was time to do things the easy way.
During Unholy Allies, one of the tie-in novels for Vampire: The Masquerade, a renegade vampiric assassin challenges one of the protagonist's allies to a duel. When another ally suggests getting involved, Madeline (another vampire assassin) promptly invokes this trope ... but does say that she'll step in immediately should their ally die.
Subverted in the Swedish science fiction novel Iskriget (The Ice War) in which protagonist Johnny realizes that it up to him to be the hero, after which decision he immediately enlists the help of a team of skilled Jewish mechanics to build the equipment he needs to execute his plan.
In the final installment of the Star WarsLegacy of the Force series, when Jacen has fully turned to the Dark Side and everyone finally realizes that cannot be redeemed, he has to be killed by his twin sister, Jaina, because she is the only one who knows him well enough to outmatch him.
Bastille: No! If we get too close to them, the power will vaporize us!
In Young Wizards, an Ordeal is something the new Wizard(s) must do themselves.
Jonas cites this trope in Son as the reason he can't aid Gabe in fighting Trademaster.
Live Action TV
Buffy the Vampire Slayer features this in the musical episode "Once More With Feeling", when Giles insists that Buffy save Dawn on her own as she is becoming too dependent on his support. The gang eventually does show up to help her out.
Later on, he does admit she needs backup.
Because she was having trouble hitting the notes of the musical number. Willow and Tara immediately rush in and start singing backup vocals.
Also, killing Caleb, although in that case it was her idea.
Subverted on the Stargate Atlantis episode "Sateda" when Ronon tells the rest of the gang that if they kill the Wraith "General", he will kill them. Ronon specifically says he wants to do this himself, as the Wraith leader had destroyed his civilization and then hunted him like an animal for seven years. He gets his ass handed to him and only survives because Beckett blows away said Wraith with a drone weapon. Ronon is actually relieved. This was a notable piece of character development, as everyone else expected him to be very angry.
Played straight with Teal'c's Roaring Rampage of Revenge. Notably, though, the rest of SG-1 was trying to stop him on at least one occasion.
In another episode, Teal'c feels weak after an injury (which would leave a human permanently crippled), especially since he now has a normal immune system (provided by the tretonin drug) instead of a larval Goa'uld symbiote with a Healing Factor. At the end of the episode, he manages to rescue his son and teacher (also father figure) from a Goa'uld penal colony and is fighting the abusive Jaffa in charge. When Teal'c son Rya'c attempts to interfere, Master Bra'tac stops him with this trope. Sure enough, Teal'c manages to overpower the guard and finish him off with a Neck Snap, looking extremely pleased with himself.
This trope is mentioned by name in "The Fifth Race" when O'Neill dials the stargate to travel to another galaxy while under the influence of the Ancient database. Teal'c wants to go with him, but Daniel says O'Neill has to go alone.
Star Trek: Deep Space Nine: In the finale, the Emissaries of the Prophets (Sisko) and Pah-Wraiths (Dukat) have to do battle for the sake of the universe in the sci-fi equivalent of the battle between God's representative and the Anti-Christ. As a result, Sisko explicitly states this is something he has to do himself before departing for the battle.
Abed from Community gets dangerously genre savvy about it (as though Abed is ever any other way):
Abed: So, how does this play out? Is there going to be a moment in this fight when I finally step up and learn to stand up for myself?
Jeff: ... maybe you should just hang back.
Abed: Yeah, okay, I will. (Beat.) IN HIS FACE!!!
Dungeons & Dragons contains the Deck of Many Things. Characters draw cards from it and receive various effects, two of which fall under this:
The Comet card will cause the character who draws it to gain a level... if he or she defeats the next monster(s) encountered single-handedly.
The Skull card makes a dread wraith (or, in earlier editions, a "minor death") appear and attack the character, who must then defeat it alone (anyone who helps gets a dread wraith to fight as well)...and if the character loses, he or she is slain forever.
Some prestige classes, especially those based on gladiator or duelist styles of combat have this as one of their prerequisites.
Warhammer has a rule for this: once two heroic characters declare a duel, only the one character and their mount may attack the other character, and vice versa.
And as of the sixth edition rules Warhammer40000 has introduced a similar rule. In both cases this covers proper characters and squad leaders.
In the game Star Fox 64, near the end of Venom, the final mission, Fox will leave behind his teammates to "go it alone" against the final boss.
The Duel Boss in Tales of Symphonia between Lloyd and Kratos is one of these, and is set up as one in advance. While either side could easily bring multiple parties into it, neither side wants to because of the personal catharsis involved. Sheena's duel with Kuchinawa is another example.
In Fire Emblem Path of Radiance, Ike decides to fight the Black Knight by himself but Mist will run in anyway if she hasn't fallen in battle. Then in the sequel Radiant Dawn, both Ike and the Black Knight agree It's Personal, so the Black Knight puts up a big wall in the middle of the battlefield, separating the two from everyone else.
Ike's weapon is the only weapon that can damage the Black Knight anyways, so it's really a Justified Trope. Also, the main characters of other Fire Emblem games attempt to go alone, but other characters insist on helping out.
Arguably deconstructed with Judith. Since her father created the deadly Hermes Blastia, she believes that she's the only one who should worry about dealing with them and tried not to get the party involved. This belief is treated as a very bad thing, and in fact causes serious problems for the party at one point.
Several Persona 2 choices can be made like this. Allowing the interested parties to settle their own accounts at the right moments will net them their evolved Personae, having transcended their issues and unlocking their true potential.
In Chrono Trigger, if you choose to fight the second battle with Magus and Frog is in your party, Frog insists upon fighting Magus by himself.
Doubly epic if you use the Masumune (and quite difficult too)
There is also Robo's fight against Atropos in Geno Dome.
In Kingdom Hearts II, during the first visit to the Pride Lands, Sora tells Donald and Goofy to hang back from Simba's fight with Scar, asserting that it's something Simba has to do himself. But when Scar comes back from defeat as a Heartless, they're happy to join in.
Used to Narmful levels in Dead or Alive 4, in Kasumi's ending. Right before the boss fight with Alpha 152, Ryu is standing there, and comments that this is something she has to do herself. And presumably he's still standing there after the jiggly jello totally wipes the floor with you, multiple times. Thanks, Ryu.
Just before the Duel Boss with Cecil's dark side in Final Fantasy IV, Cecil tells the Twins and Tellah to "Stay back. This is a fight for me, and me alone."
All over the place in Dissidia: Final Fantasy, the heroes always battle their villain alone. If they're accompanied by allies, like Terra and Bartz are, they will directly tell their allies "I'll handle this" and fight alone.
A similar case happens with Baret in Final Fantasy VII where former friend Dyne attacks the party and Barret tells his friends to back off because this is a fight he has to deal with by himself.
In one attempted subversion, Squall is attacked by both Ultimecia and Garland, and resolves to fight both of them alone, only for Zidane to charge in and tell Squall to stop showing off and take care of Ultimecia.
Played with in Siren. Shiro nearly quotes the trope verbatim as he goes to finish off the Onda sisters. However, by this point, he has murdered his twin brother and assumed his identity, making the whole 'Things you have to do for yourself' bit a little ironic...
Buki of Sudeki takes on a giant spider boss monster by herself because "Like me, he is a Shadani Warrior" (this was actually a game design decision — bosses simply worked better against lone characters)
Only one boss is fought with your full party, and he acts more like a King Mook than the rest of the bosses. He also brings in swarms of Mooks to back him up.
In Fallout 3, this is forced on the player at the end, when you're asked to make a moral choice to either save the world and get exposed to lethal radiation, or chicken out and have a heroic woman do it for you. At this point it's possible to have a Super Mutant follower who is completely immune to radiation. If you ask him to do it, he just says that the cunundrum is something you have to work out for yourself.
Thankfully averted with the aid of DLC.
Yet you still get called a coward for getting someone else to do it. Luckily, you still survive if you do it.
The Extended Cut of Mass Effect 3 has Shepard order away his/her squadmates to safety, while they make the run to the portal leading to the Citadel by themselves.
In Breath of Fire 2, it is possible to choose to solo the fight against Barubary, a demon who is the cause of much trauma in Ryu's childhood. The fight is pretty difficult even with a full party, but your reward for beating him with Ryu alone is an obscure hint as to the location of treasure in his dungeon.
CJ invokes this trope on Sweet in the final mission in Grand Theft Auto: San Andreas. CJ insists he takes on the Big Bad alone in order to make up for running out on his gang when they needed him most.
Subverted in The Order of the Stick, where Hinjo calls Roy off so he can try to reason with fallen paladin Miko. That goes about as well as you'd expect, and Miko is about to kill him when Roy says "Screw this" and swats Miko into unconsciousness.
Tarvek: So, Wulfenbach—just checking—is this going to be some kind of macho exercise where you insist on battling a potentially superior opponent alone—in some kind of attempt to "prove" your intrinsic worth?
In Sonichu issue 9, the titular hero is facing down Big Bad Slaweel the Witch and handily defeats her, leaving her hanging off the platform they were fighting on. However, he ends up saving her and lets her get away because only Chris can defeat her. Which he promptly does in the next issue.
Meji: Um... he did know that I could just as easily have blown up their entire camp for him instead, right?
Sarine: Quiet ... he needs this, I think. If nothing else, at least it'll probably make him stop whining for a little while.
In the novel Exams of the Whateley Universe, massively over-powered characters Tennyo and Fey insist on Jade fighting two (mediocre) supervillains herself, to prove something important about Jade to Jade: she's not really cut out to be a pacifist.
Dragon Ball Z Abridged has a memorable subversion. When Tien's best friend/dojo mate is killed by Nappa, Tien attacks furiously to try to avenge him... and promptly starts getting his butt kicked by Nappa. Gohan, watching from the sidelines, asks his mentor Piccolo if they should help, but Piccolo replies that this is something Tien has to do himself. We then cut back to Tien, who is getting a No-Holds-Barred Beatdown from Nappa and is frantically screaming for help.
Lampshaded and subverted during a Global Guardians third year story. Achilles, leader of the titular hero team, discovers the bad guy who has been causing them so much havoc is his own brother. Cue Gunmetal asking "Is this going to end up one of those 'I have to face him alone' things with you?" To which Achilles responds, "Oh hell no. Take his ass down the moment you can get a clear shot."
In one episode of Care Bears: Adventures in Care-a-Lot, Grizzle manages to steal Funshine's belly badge and uses it to try and take over Care-a-lot. Funshine tricks Grizzle and gets his badge back to prove the episode's aesop, but the fact remains that there were myriads of still-badged Care Bears who didn't lift a finger to help him.
While everyone did acknowledge that defeating Ozai was that Avatar's responsibility (simply because if anyone else killed him, it would be seen as an act of rebels/fighting for the throne, rather than divine intervention), no one (not even Sokka's group) was actually CLOSE enough to have helped if he needed it.
They were also severely outclassed by comet-empowered Ozai - Aang and Iroh were about the only ones who stood a chance in the first place.
Still, it's not like the rest of the gaang just sat down doing nothing while Anng fought Ozai: Zuko and Katara were fighting Azula, Sokka, Suki and Toph were sabotaging the Fire nation fleet while Iroh and the White Lotus were reconquering Ba Sing Se.
When a few members of the Justice League end up in the Old West, Jonah Hex prevents them from getting between the local town's hero and the main bad guy, because, in Hex's words: "A man's gotta settle his own accounts".
In The Avengers: Earth's Mightiest Heroes "Panther's Quest", Black Panther insists Wakandan tradition says he must defeat Man-Ape by himself. Captain America respects this, but when Man-Ape orders the Dora Milaje to take out the Panther, Cap intervenes.
Cap: You can face Man-Ape alone. Anyone else is fair game.
Recess: In the episode, "The New Kid", Gus has his name taken away from him by King Bob and is only allowed to be referred to as "the new kid." Wanting to cheer him up, TJ and the gang get the whole school to celebrate Gus' arrival at his school and his name, but immediately stop when King Bob's entourage come by. At this point, TJ tells Gus that if he wants to be acknowledged as a human being and not just as "the new kid", he'd have to stand up to King Bob himself.
Season 3 premiere of My Little Pony: Friendship Is Magic had this for Twilight, who must retrieve the Crystal Heart to save the empire. Subverted in the end, as Twilight got trapped and had to get Spike to finish the quest instead.
The origin of this trope is, perhaps, more historical than legendary. During the Battle of Crécy, King Edward III of England left his son, the Black Prince, and his retainers without reinforcements so that he could "win his spurs, for I want him, please God, to have all the glory." The Prince survived and was gifted with the crest of the defeated King of Bohemia.