Takes One to Kill One
aka: Only Like Can Cut Like
"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
, 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?
A subtrope of Mutual Disadvantage
. An opposite of Tactical Rock-Paper-Scissors
where each type of combatant has some other capable of curb stomping
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.
- Same goes for its spin-off series Blood+. Chiroptera will regenerate from basically any wound. Only the blood of an opposing queen can put them down. Same goes for everything else spawned by that Queen, and the Queen herself for that matter.
- 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 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.
- 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
- 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.
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.
- 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.
- Hoopa, a recently discovered Pokémon with a Ghost/Psychic type combination, has the honor of being the only type combination with a double weakness to one of its own types! note
- 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.
- 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.