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Having only one meaningful square-peg ability and trying to apply it (often with surprising success) to round-hole enemies is When All You Have Is a Hammer…, which is probably what you're looking for. Gone too far (for comedy), The All-Solving Hammer.
If Power A really is that versatile and it wouldn't feel right to call it a hammer, then Swiss-Army Superpower.
I was looking more for examples where it is partially (or at best, back-asswardly) effective. There are tons of them in fanfic but there seem to be relatively few in canon works. Thanks for responding.
Seconding When All You Have Is a Hammer… - one of my go-to examples is when this combines with Beyond the Impossible, and you get a brute-force Idiot Hero who manages to punch a spell to death because he's too stubborn to stop trying.
Edited by Scorpion451
Agreeing with When All You Have Is a Hammer…, citing With This Ring: chapter
"I suppose when all you've got is a hammer, you learn to get really good at fixing things with a hammer."
On the other hand, "using mundane hypnosis to speak to a magically created alternate personality" doesn't fit this concept, as it is just as 'effective' as "using mundane hypnosis to speak to a mundane created alternate personality".
Link to TRS threads in project mode here.
While it for sure overlaps with When All You Have Is a Hammer…, I do want to reiterate that this is where the hammer is either only semi-effective — or requires way more work than doing it with the right tool (so to speak) would have taken. I posit that there's enough difference in concept to justify a separate trope.
The When All You Have Is a Hammer… page even has a perfect example of what I have in mind: the Shirou vs Blob Monster from Fate Gamer Night.
Contrast that with most of the rest of the page where it's all examples where the hammer either works perfectly or someone is prevented from using the hammer.
Yes, the example you point out is an ideal example of When All You Have Is a Hammer….
I think what you're seeing in many of the other examples is the tendency for the trope to slide into The All-Solving Hammer over the course of longer works via Serial Escalation. Unless you're writing an extreme Failure Hero learning to accept that they just have a useless power, basic storytelling requires that they figure out more creative ways to apply the hammer, or at least get better at swinging it.
For instance, your example has just learned that it is possible to destroy blobs with a sword- so the lesson he has learned is not to use methods other than the hammer, but that next time he can start slashing his way to the core immediately and save some time and effort.
Edited by Scorpion451
THE LESSER HAMMER
Looking for a trope where Power A is used to handle Situation B and is semi-effective because it's not really ideally suited (EDIT: either because it's poorly optimized but not useless or because it takes way more work than using the exact right tool for the job) but it's what you've got.
An example would be Metamor City where Brian Sommers uses lightning bolts against vampires when what he really needs is fire or Buffy the Vampire Slayer where regular bullets are used to shoot vampires ("Yeah, it won't kill you but it'll hurt like hell.")
Non-vampire examples include stuff like Nezuko kicking the head off a demon with sheer brute force instead of using a proper Nichirin blade or Anasurimbur Kellhus breaks a magical geas on someone's mind by using mundane hypnosis to speak to the magically created alternate personality enforcing said geas.
EDIT: See also Game of Thrones where fire is useful against White Walkers but sub-optimal when compared to dragonglass.
EDIT: Also also also, Ranma fic ''The Clan" where where he fights demons that regenerate everything except self-inflicted wounds — by Deadly Dodging them into traps they can only escape through self mutilation. Meanwhile the "right" way to kill them is by inducing suicide with vampire mind control eyes.
Basically Wrong context powers forced to work well enough, round-hole enemies defeated by square-peg abilities, inefficient but effective.
September 29 2023 EDIT: in The Warded Man, demons with a sunlight allegy rise every night and kill anyone who isn't protected behind magical defensive wards. A particularly brave young man accidentally figures out how to use defensive wards offensively (by closing the ward circle right when the demons arm is across it thus severing the limb.) He also stumbles onto an entire warrior culture that does similar tricks (luring demons into closed circles of defensive wards and trapping them till sunrise.)
They do eventually discover offensive wards but up to that point, this would-be trope fits.
Edited by RazorSmile