Takes One to Kill One
"Magic must defeat magic!"In many works of fiction, there are creatures that are Immune to Bullets, Made of Iron, and in worst-case scenarios have no Kryptonite Factor. Even nuking them has no effect. But fear not, for while these creatures may be immune to fire, bullets, nuclear weapons, and MTV, these monsters are still have one weakness...another member of their own kind. This is often used as a justification to why the protagonists, who either are this kind of monster, can turn into one, or have one under their control, can seriously wail on the Monster of the Week, and yet the military forces and police are still useless - though if this becomes a trend, it may lead to other characters being unable to effect the plot in a meaningful way or even turn those connected to the monsters into a Spotlight-Stealing Squad. If the monster is unaffiliated with either side, it can lead to Always a Bigger Fish. If the protagonists are forced to become this type of monster to win, expect a My God, What Have I Done? moment afterwards. A subtrope of Mutual Disadvantage. An opposite of Tactical Rock-Paper-Scissors where each type of combatant has some other capable of curb stomping it. Compare Beat Them at Their Own Game, Hoist by His Own Petard and Summon Bigger Fish. Contrast Like Can Not Cut Like.
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Anime and Manga
- In Neon Genesis Evangelion, the Evas (who are pretty much Angels anyway) are usually the only things on Earth which can fight an Angel and win, because only they can break the Angel's AT fields. To be more accurate, Angels project AT fields so strong that only the similarly-sized Evangelions possess the power necessary to breach them. There are cases where the Evas weren't directly responsible for the victory. One was beaten by blowing up a destroyer in its mouth, Ramiel was taken down by a positron cannon fed with the entire country's power supply (wielded by an Eva, but the cannon did the work). Certain other options exist to hit the 180 million megawatt figure stated to be necessary to overcome Ramiel's AT-Field, which was one of the strongest of all the Angels (behind Tabris and Zeruel, most likely). The largest nuclear devices, for instance, could likely knock out an Angel, though nuking them isn't exactly an ideal solution.
- In The Big O one of the few things that can damage a Megadeus is another Megadeus, though this is less an issue of compatibility than a matter of sheer firepower. There have been several organic threats which could have legitimately beaten a Megadeus.
- Ryo Takatsuki's nanite-powered Super-Powered Evil Side in Project ARMS has a unique power to negate the Healing Factor of other ARMS.
- Blood The Last Vampire has this as its basic premise.
- The plot of Eternal Sabbath revolves around getting Shura to kill Isaac, as Shura is the only one who understands Isaac enough to stop him.
- In Yu-Gi-Oh! ZEXAL, the Numbers monsters cannot be destroyed in battle except against another Number. Subverted in that they can still be destroyed by card effects, and it's entirely possible to negate their invulnerability so that they can be destroyed by any monster.
- In Baccano!!, the only way an immortal can die is to be absorbed by another immortal.
- Puella Magi Madoka Magica seems to play with this trope. At first, the Magical Girl magic appears to be the only way to kill Witches (straight trope), but it is eventually revealed that Akemi Homura actually uses firearms and bombs (subversion) to kill Witches off, because of her comparatively weak offensive powers. And then, in episode 11, she blasts Walpurgis Night with literally thousands of explosions and doesn't even manage to scratch her (keep in mind she usually manages to kill witches off with barely one hundredth this firepower). For all intents and purposes, it turns out Walpurgis Night can't be stopped by either firepower -or- magic, and the only reason she is defeated is the Reality Warper power of Madoka's wish which causes a Cosmic Retcon to erase all witches from existence altogether. (thus, double subversion).
- In the Madoka RPG on the PSP, where the player may keep the entire cast alive by the end, rather than only Madoka and Homura, it turns out Valpurgis can be beaten in a straight fight.
- Since, in a cruel double subversion, it turns out that Magical Girls ultimately always turn into witches if they're overcome by despair or run out of energy, YMMV as to whether this can considered a better ending than the original since Madoka didn't get to make her wish.
- In Heroic Age, all Nodos are so exceedingly powerful that the only thing that can fight a Nodos is another Nodos. Anything else is certain to get curb stomped. Even then, battles between Nodos can go on for quite a while. At one point, a Nodos battle lasts for 300 hours straight.
- Invoked in YuYu Hakusho by Kurama when he and Hiei fight against a robot made of an "indestructible" metal. Kurama leads its stretchable claws to crash into each other, which breaks the metal apart and allows him to destroy its exposed internal components.
- Never specifically mentioned, but in Haruhi Suzumiya, the only way the reality-warping, knife-nut Ryoko Asakura is defeated is when Nagato intervenes. No one else would certainly be able to do it.
- In Claymore, the eponymous warriors are humanity's only hope against the yoma—because they are infused with yoma blood to gain control over yoki, their demonic energy. On the flip side, all Claymores run the risk of turning into super yoma, a.k.a. Awakened Beings. However, the beings the Claymores were really meant to fight are the Dragon-people whose flesh is the source of yoma.
- Attack on Titan: It's revealed that the only thing that can reliably hurt a Titan is another Titan. Which makes the Titan Shifters extremely valuable allies.
- In Fairy Tail, the only magic that can even scratch Dragons is Dragon magic. Dragon Slayers are simply humans who wield the same kind of magic as Dragons. The only exception to this are Demons who rely on "Curses" which are fundamentally different from magic. Even then, the Dragon struck by the Curse was already dead, being a spirit.
- In JoJo's Bizarre Adventure, a Stand (an invisible telekinetic construct) can only be harmed (inflicting sympathetic damage on its creator) by another Stand. Although some Stands can't be harmed at all.
- Zig-zagged in the Gundam franchise: Sometimes Gundams can only be defeated by Gundams, while other times any sufficiently advanced mobile suit can potentially defeat one.
- The director of Mobile Suit Gundam SEED outright said that "Only a Gundam can beat a Gundam" was part of the series' design philosophy. In this case it was aided by the fact that most Gundams use Phase Shift Armor, which is immune to physical damage, meaning that only beam weapons (like those wielded by Gundams) can hurt them.
- In Mobile Suit Gundam 00, Celestial Being's Gundams are several orders more advanced than the forces of Earth's military, particularly in their use of the GN Drive solar furnace which grants nigh-infinite power. Late in the first season, the Earth forces are handed a supply of GN Drives and a mobile suit designed to use them, which results in their finally being able to fight the Gundams on equal footing. By the time of the second season, the new Earth Sphere Federation has mass-production machines that outclass the original four Gundams in a straight fight...which, conveniently, is when the Mid-Season Upgrades roll out.
- A variant is used in Tokyo Ghoul. Ghoul Investigators hunt Ghouls using a weapon called a Quinque, which is made using a Ghoul's kagune — one of the few things capable of harming a Ghoul. The audience learns this when Kureo Mado brings out his Quinque to kill Mrs. Fueguchi, who recognizes it as having been made from her husband. Later on, he brings out his newest Quinque to attempt to kill her daughter — he had it made from Mrs. Fueguchi after killing her.
- Superman's immunity to bullets and most other forms of bodily harm come from an innate forcefield he projects around himself. However, other Kryptonians are able to pierce this forcefield and harm him.
- When the New 52 version of Supergirl first fights the Worldkillers, one of them boasts that only a Worldkiller can defeat a Worldkiller. Supergirl exploits this by grabbing one Worldkiller's Combat Tentacles and stabbing another one with them.
- In Les Légendaires, Anathos reveals in his final stand against the Legendaries that only a God can kill another God. Since he is at this point the only God left on the planet, the only option for them to kill him would be if he committed suicide. They end up defeating him by stabbing him with the sword of his host Danael; since said sword was made from Danael's (and as such Anathos') blood, it's technically part of him.
- In The Incredibles, Mr. Incredible finally realizes that the only thing that can defeat the Killer Robot is the robot itself. Specifically, its detached claw that he can fire back at the robot, after taking the control brace away from Syndrome.
- This is also justified in-story because Syndrome was constantly testing newer models of the Killer Robot against seasoned superheroes, providing new modifications to overcome nearly anything each time (many heroes would defeat a model, only for him to create one with an immunity to that). By the end, it was so strong that only its own claws seemed capable of destroying it.
- Ninja 3: The Domination
"Only a ninja...can destroy a ninja."
- In virtually all kaiju films, especially those with Godzilla and Gamera, the only thing powerful enough to take down a giant monster is... well, another giant monster. However, the other monster can be manmade; as Pacific Rim puts it, "To fight monsters, we created monsters of our own."
- In Mega Shark vs. Giant Octopus, the entire U.S. Navy couldn't take down one of these creatures. The plan? Have them kill each other!
- In The Wolverine, Wolverine's adamantium claws are severed by a superheated adamantium blade wielded by the silver samurai.
- Nicodemus, one of the Rogues Gallery of the Dresden Files, is protected by the noose that hung Judas, which regenerates damage from any cause... except for itself.
- In Dead Beat, only someone surrounded by necromantic energies could approach the nexus of the Darkhallow and live. The only way anyone but another necromancer could get near it was to use necromantic energies on something non-human, which technically isn't forbidden. Luckily, Harry just happened to be near a tyrannosaurus skeleton when he found this out...
- In the Arcia Chronicles, the Arc Words in The Prophecy of Eric, "Darkness will protect from darkness; light, from light," foreshadow the protagonists' discovery that the best way to combat dark or light magic is with more dark or light magic, respectively.
- In Harry Potter and the Half-Blood Prince, when the new (unnamed) Prime Minister is warned by the new Minister for Magic (Rufus Scrimgeour) that Voldemort is on the move, the PM remarks that the Ministry should be able to handle him since "you can do magic." Scrimgeour reminds him that Voldemort and his people can do magic too.
- Percy Jackson and the Olympians: Beings such as gods and monsters can be hindered by mortal means, but not seriously harmed or destroyed. As such, most heroes carry weapons made from divine metals like celestial bronze, which can both kill monsters and prevent casualties by passing through mortals like a ghost. Demigods, being somewhere in between, are vulnerable to both mortal and divine weapons.
- In The Hollows by Kim Harrison, law enforcement generally sends the same species as the criminal they're dealing with. They know all the ins and outs of how they work, and can defend themselves against their usual tactics. So vamps are sent after vamps, witches after witches, banshees after banshees, etcetera etcetera. Playing mix and match tends to have...poor consequences.
- In the Sword of Truth series, the D'Harans say that they are "the steel against steel so that the Lord Rahl can be the magic against magic." Since only the Lord Rahl has the ability to combat magical threats, everyone else has the duty to combat physical threats (and they tend to get concerned when he tries to deal with them himself).
Live Action TV
- In Angel season 4, Angelus figures out that the only thing that can harm the Beast is a knife crafted from its own rocky hide. Which it had conveniently crafted already as an offering to its master.
- One of the spells in Charmed for defeating a ghost only works if it's said by another ghost. Fortunately, becoming a ghost doesn't require that you actually die, just be unconscious and near death.
- In Supernatural, according to lore, the only way to kill a dragon is with a sword forged with dragon's blood.
So you need one to kill one, but you got to kill one to make one. How does that work out?
- The only way to permanently kill Leviathans is for them to be eaten by other Leviathans. They can be killed via decapitation, but they only stay dead as long as the severed head is kept separate from the rest of the body. If the head is left close enough to the body, it will reattach and the Leviathan will come back to life.
- Before the Leviathans come into the picture, the only sure way to kill angels is with an angel's blade. Archangels take it even further, as they can only be taken down by an archangel's blade.
- One episode of Xena: Warrior Princess dealt with a villain making weapons out of Hephaestus's metal, which can only be damaged by other weapons made of Hephaestus's metal. Including, it turns out, Xena's chakram.
- Older Than Feudalism example: In a poem ascribed to Theocritus (3rd century BCE), Hercules discovered that the only thing that could pierce the Nemean Lion's hide was its own claws. Hercules had already strangled it to death by this point — he just wanted to skin it.
- In an undated Greek myth, the twin giants Otus and Ephialtes were so strong they could only be harmed by one another. Artemis tricked them into shooting one another with their spears, killing both brothers simultaneously.
- A lot of werewolf and vampire fiction states that one of the few things werewolves and vampires are vulnerable to (except, you know, silver, stakes, and such) are bites and scratches inflicted by another werewolf or vampire.
- Alucard and Blade are both vampires who hunt vampires.
- Van Helsing: The only thing that can kill Dracula is a werewolf.
- In the Old World of Darknessuniverse, werewolf claws do aggravated damage, meaning that they are one of the few ways to easily take down another werewolf or any variety of supernatural beasties.
Table Top Games
- In Dungeons & Dragons 3rd edition and 3.5, creatures with damage resistance have their natural weapons (if existing) able to pierce that same resistance, so every creature is capable of killing its own species.
- In 4th Edition Dungeons & Dragons, The Prince Of Hell Epic Destiny gets immunity to fire. He also gains the ability for his fire attacks to ignore resistances / immunity to fire, meaning only he can burn himself.
- In 1st Edition, before serious attempts were made to balance it with other game elements, psionics was something that non-psionic opponents had little or no defense against. You pretty much had to use psionic monsters and enemies to keep a psionically-gifted PC from running roughshod over a campaign.
- In Werewolf: The Apocalypse, one of the setting ending scenarios has the Wyrm having become so powerful that only its own weapons can even hurt it.
- In Deadlands, The Dragon to the Reckoners, Stone can only be killed either by shooting him with bullets that should've killed him on Gettysburg, some of which are in his guts or by any weapon shot by his own hands.
- In Breath of Fire: Dragon Quarter, dragons can only be harmed by other dragons. This means that if Ryu transforms in a conventional battle (including the vast majority of boss battles), he is invincible. There is one exception: A certain enemy dragon can be harmed with conventional weapons if a transformed Ryu attacks first and "breaks the defense".
- Magus in Chrono Trigger, a Barrier Change Boss, can only be damaged by the element he's currently using.
- Iji was enhanced with nanomachines in order to combat the threat of alien invaders, because nanoweapons are about the only way to even scratch the alien's armor, much less go up against one and win.
- And the final boss must be taken down with his own projectiles.
- Similarly to Iji above, the last boss of Sonic 3 & Knuckles must be damaged with it's own missiles: despite the fact that you are invincible, ramming the boss not only does no damage, but wastes the precious little seconds you have.
- In Persona 3 and Persona 4, Personas are actually Shadows tamed and controlled by a sentient being's ego (with Shadows themselves being the coalesced feelings of despair and loathing in all people, so it can be said that Shadows are the Persona of humanity). As such, they can (and have) turn back on whomever they're a Shadow of and consume them, but are also the most effective way to fight Shadows.
- Perhaps rather famously, the Dragon type in Pokémon, though it is also weak to Ice. Generally, this a better bet anyway, since fighting dragons with your own dragons makes them into Glass Cannons, naturally.
- Same with the Ghost type, though they're vulnerable to a number of other types as well. Since Giratina is of both types, it is often the best counter to itself.
- In the first generation, the Psychic type, while resistant to itself on the Elemental Rock-Paper-Scissors tree, was such a Game Breaker that the only reliable counter was another Psychic. This was severely nerfed for generation 2.
- When you start messing around with the Elemental Rock-Paper-Scissors, you may start getting combinations of types that wouldn't normally resist themselves paired with a type that is weak to said type- like how the Fighting/Steel Lucario is weak to Fighting, the Bug/Grass Parasect is weak to Bug, and the Steel/Ground Steelix is weak against Ground. note
- "???"-type enemies in Alter AILA Genesis are the enemies who use Psych-elemental attacks the most often, but they are also the only enemy weak to it.
- Many other enemies do this too; robots use Shock attacks the most, and humanoids tend to use Force, both of which are the elements they're most weak to.
- Metal Man from Mega Man 2 can be one hit killed by its own weapon.
- In Final Fantasy Tactics, the "Faith" stat dictates not only a mage's attack power, but how much damage they incur from enemy spells as well. A character with very low Faith is thus virtually immune to magic, while a character with high Faith is very vulnerable.
- Low Faith isn't quite as good as it sounds, because it makes the character virtually immune to all magic, including healing spells. So a character can be impervious to magic, but only at the price of having to rely entirely on items for healing. Though it's actually not a bad trade-off in this game.
- In Paper Mario: The Thousand-Year Door, during the Inevitable Tournament, one of your enemies is a pair of metallic, spike-covered critters... you can't hurt them with jumping-attacks, hammers, fireballs, or anything else. In fact, it's a Hopeless Boss Fight... until Yoshi Jr. joins you for the rematch. By eating one and spitting it at the other, he's capable of damaging them - because, as it turns out, their only vulnerability is themselves.
- Dragons of Skyrim are otherworldy beings bound to flesh. Upon death, their souls do leave their bodies, but can easily return with help from another, making them functionally immortal. The only way to truly kill a dragon is for another dragon to devour their soul. The player character, a mortal born with the soul of a dragon, is close enough to a real dragon to pull this trick off themselves.
- This is how the Cerebrates are defeated in Starcraft. "For the Dark Templar use energies that are much like my own, and it is by these energies that they have caused me harm."
- Similar to Pokémon, the Dragon element in the Monster Hunter games is the best one against Wyverns.
- The Siege Engine in Warcraft III is basically a steam-powered battering ram: it deals high damage to buildings, can't attack ground units, and has building-type armor... which is particularly weak to siege-type damage, which all factions' artillery have. Downplayed, in that killing it with normal units is only a bit longer.
- In Homestuck, it's believed that one way to kill a nigh-invulnerable First Guardian (or a creature possessing its powers) is to send another to fight it.
- Courage the Cowardly Dog had the Kangaroo Monster, a creature so Badass the only thing that could harm it...was another Kangaroo Monster!
- Jackie Chan Adventures: "Magic must defeat magic!" Magic Versus Science plays something of a role in the series where Magic A Is Magic A, but the main problem is that trying to use Science B in the equation produces either null results or Unpredictable Results. The biggest example is when Jackie uses a laser to destroy the Chinese zodiac themed talisman's in an effort for No Macguffin No Winner. While the physical talisman's were destroyed, the magic within them transferred to a corresponding animal, forcing them to search for the talisman powers again in a different form.
- On South Park the only way to defeat a giant stone Abraham Lincoln is with a giant stone John Wilkes Booth.
- Also, immortal beings like Cthulhu can only be killed by other immortals.
- In a similar situation in Dexter's Laboratory, Dexter uses a giant stone Abraham Lincoln (who shoots missles out of his hat!) to fight Mandark's giant stone George Washington. However, the trope is subverted when the two stone presidents find out they are honest men who have a lot in common, stop fighting, and walk off as friends.
- In another episode Dexter gives Dee Dee what he thinks is a silencing potion, but instead mutates her into a monster. The only way to get her to shut up? To become a giant monster himself and have a knock-down, drag-out kaiju battle!
- Parodied in a "What If" episode of Futurama, where the Planet Express crew creates a Giant Zoidberg as only an "even equally big" monster could defeat a tall monster rampaging through the city. This fails miserably as Giant Zoidberg immediately goes on a power trip and rampages through the city, and so the two giants fight and kill each other in the ensuing turf war.
- In the second movie, The Beast With A Billion Backs, electromatter can only be damaged by other electromatter.
- Ultimate Avengers provides a technological example: vibranium armor can only be penetrated by vibranium weaponsnote .
- In the ThunderCats episode "The Ghost Warrior", the ghost of the evil warlord Grune the Destroyer awakens from his ancient slumber and goes on a rampage. The ThunderCats try to fight him, but nothing they try can hurt him, while he can hurt them. Eventually, Lion-O realizes, "We fight a ghost... with another ghost!" and summons the ghost of Jaga, who defeats Grune and banishes him back to the afterlife.
- Gerry Anderson's New Captain Scarlet introduced a Russian super-tank the size of a small apartment building called a Druzhnik, which was set up as being pretty much entirely untouchable by conventional weapons; a GDI Mammoth Tank might've had a chance in a 1v1 but the heaviest weapons SPECTRUM had access to just bounced off it. The third time the Mysterons managed to swipe one, in the grand Gerry Anderson tradition of Prop Recycling, Scarlet pretty much quoted the trope word for word:
"Nothing can touch a Druzhnik, except for another Druzhnik!"
- He then proceeded to prove his point, having borrowed one from the Russians. It was rather badass.
- British forces in the Battle of the Atlantic were commanded by a submariner.
- Frequently inverted by the actual real life military. Often, the best way to kill or disable a hostile unit is using something completely different against it; for example, battleships fell out of favor after WW2 because too many of them had been destroyed by aircraft, and a tank's worst enemy isn't necessarily always another tank.
- That's often because the best use of tanks is in destroying supply lines, rather than because tanks are not effective against other tanks.
- Averted with submarines, Hot Sub-on-Sub Action notwithstanding. WW2 and earlier subs, largely blind underwater and equipped mainly with unguided torpedoes, were basically incapable of attacking each other at all unless the target happened to be on the surface (in which case it would have been just as vulnerable to gunfire and aircraft), and while more modern ones are equipped with weapons that can seek out and destroy another submarine those weapons can often be just as easily be mounted on other platforms (such as helicopters, against which a submerged sub under attack cannot even return fire).
- Diamonds can only be scratched by other diamonds.
- Or by even harder substances, the first of which, cubic boron nitride, was synthesized in 1957.