Hiya, need help finding a TV Trope…
In Sci-fi stories it’s normal for the baddies to have ridiculously powerful weapons and armor. Often they are only vulnerable to their own weapons, eg. in “classic” Doctor Who (1963-1989) the Cybermen are hard to kill but can be shot with their own captured weapons. Same with the Daleks.
What’s this trope called? Thanks 8^)
Thanks for the suggestion but I’m not sure that’s quite it - that refers to a plan or weapon being the cause of a villain’s downfall. I really meant that baddies are invulnerable to normal attack until one of their own weapons is captured and used against them. That is to say, bullets etc don’t work but their own weapons do. It’s a common contrivance in the Doctor Who of old.
I think Waldo’s got you pretty close there. Look at the related and sub tropes listed on the page, perhaps, to zero in on what you’re looking for.
I know; they specifically mention the Daleks being shot by their own weapons in that entry. They also mention the comic-book example of Darkseid, who can shrug off all sorts of attacks but gets brought down low by his own; the TV example of a demonic villain on SUPERNATURAL getting killed by her own demon-killing knife; the cartoon example of Baxter Stockman’s impressive power armor being no good against its own severed arm cannon; the video-game example of Kratos taking the Claws of Hades away from Hades for to use them against that god for the win; and so on.
I guess I’m not explaining myself properly, sorry!
An example of what I mean is in the 5th Doctor Story “Earthshock” where the overall plot is concluded by the Cybermen accidentally sending the freighter back in time 65 million years and inadvertently causing the extinction of the dinosaurs and the beginning of homo sapiens. That would be an example of “hoist by their own petard” as they wanted to blow up the Earth but their own scheme defeated them. Same as the Dalek examples on the Tropes page eg. Davros defeated because he removed pity from the Dalek’s consciousness.
What I mean though is that the Cybermen throughout Earthshock are invulnerable to normal attacks but during the story a few of the Cybermen weapons are obtained by the protagonists and used successfully against the Cybermen in small skirmishes. So what I mean is that as superior creatures / aliens / whatever normal guns don’t harm them but their own firearms do - prompting the Brigadier to wish “You know, just once I’d like to meet an alien menace that wasn’t immune to bullets.”
Something about monsters being magic super-beings but bizarrely designing enough firepower to kill each other. Kinda silly in a race of robots all acting as one. Thanks :^)
It is the “Takes One to Kill One” trope.
FYI, in the second Doctor episode “The Moonbase” (at least in the novelization), the Doctor’s companion Polly makes a compound that’s basically nail polish remover which can kill a Cyberman if poured onto its chest. and in “Revenge of the Cybermen” gold can kill them if it gets into their chest. Even in" Earthshock", didn’t someone kill a Cyberman with Adric’s gold-edged medal for excellence in mathetmatics?
Nice one, thank you rainy!
And I think you’re right Dendarii Dame weren’t they vulnerable to gold in Revenge of the Cybermen, and to Ace’s gold coins in Silver Nemesis… Always good to have a backup plan just in case!
Thanks everyone who replied 
Sp Perseus using the head of Medusa (a titan) to take down the Kraken (another titan) would be an example?
Could be. I’m also reminded of Mr. Incredible and his family using the detached, rocket-powered, ultra-hard hand of the Omnidroid to punch a hole through it and destroy it.
Alrighty, since we’ve solved the OP’s trope question, how about answering this one: A soldier/vet-- even a low-rank grunt-- when found in the civilian world, has the ability to do absolutely anything. Land a plane, amputate a leg, stitch a gaping head wound, contain and treat an outbreak of TB, rig homemade explosives to take down an entire enemy camp, hack into DoD classified emails, defuse a nuclear hand grenade, crack a safe, navigate an out-of-control cruise ship full of babies away from an approaching waterfall, become the wise leader everyone looks to AND use South American twigs and berries to create an antidote for a venomous snake bite on his lady’s gorgeous, yet dirt-speckled, leg.
I’ve come to refer to these characters as the Magic Military Man, but I haven’t been able to find any “official” trope listing on this type of character. An example would be the Barbie character on “Under the Dome.” Is there an actual name for this type of character/trope?
Lazarus Long?
You may also want to look at Master of All and Renaissance Man.
Seems to be the Do-Anything Soldier trope – where they mention, for example, the JAG lawyer who’s also a fighter pilot and gets hailed as a “credit to the uniform” while ably undercover as a Force Recon Gunnery Sergeant in the USMC.