So yeah, why Aya and Keine are awesome pair to destroy Gensokyo? Obviously, because Gensokyo is land of FRAUDS, and to win you need to be even bigger fraud. Aya, supposedly holds only source of mass media in her hands that villagers consume. Back in the day Hecatia herself asked Aya to not publish her inconvenient newspaper, so it's safe to say she's opinion leader of present. Keine's ability is to literally hide history, and once per moon cycle to literally erase and make up new history.
Who controls past and present also controls future, so with Aya and Keine we have ideal combo to manipulate public opinion however we want.
Total youkai genocide will go as such:
0) Keine hides dangerous actions and material evidence with her ability full time from all other youkai (her power affects them just fine including Reimu and Yukari as it was shown in IN).
1) Aya starts running popular science/scientific atheism column in her newspaper, explaining how certain youkai type aren't real and giving alternative explanations to whatever fears villagers have with this youkai species, starting from least relevant one by one. Keine starts teaching natural science during her classes too, preparing completely new generation.
2) Once fear and acknowledgement among humans towards certain species reduces and it's full moon, Keine in her EX form erases any evidence of that species completely.
This way, most of the cast can be erased, even if it takes years. Only ones that cannot be erased this way are youkai who live off not from fear/faith (Yukari and Remi) and who live/come outside of Gensokyo (lower than old hell and higher than youkai mountain).
3) Wait 10-20 years, during these years on full moons Keine bit by bit adds technological stuff and knowledge about it from Meiji era like tractors and stuff. Aya starts reprinting adapted newspapers from that time and villagers now know they are actually japanese with emperor "somewhere there" and shiet. Some more modern things can be stolen from Rinnosuke as well.
4) ??????????
5) PROFIT!
By the end of this time, village looks like super poor rural settlement from outside world, with not as critically outdated mindset.
4) New generation grows up. It grew up educated in relatively modern fashion, was taught real Japan's history to this day, haven't ever seen or interacted with youkai. Moriyas went extinct completely by this point so there's no point to go to youkai mountain and see kappa/tengu.
5) Here comes final blow: just before yet another full moon Aya "reveals" in evening newspaper that all supernatural shit yet unexplained has been part of shady government experiment, and youkai mountain was just secret military base, and the moment full moon shows Keine uses moment of doubt to overwrite kappa and tengu according to this story. With no youkai threat Hakurei faith plummets (Reimu is known fraud already anyways), barrier falls. All lesser youkai and gods are dead, buddhist/taoist sects lost their magic. Yukari and Remi still can nab people here and there and live off it together probably, but Gensokyo in general is dead.
>>1520153
> Do they need to know?
Well, to actually believe it's end of the world and that it's explosion that lead to winter -> "we're doomed and resistance is futile". Otherwise it would be shock, but not critical and everyone will think on Letty quite fast and slap her ass.
> explosion wipes out most the the population
This one depends on how big you imagine human settlements are in Gensokyo, but even with small estimate of 500 in population, it would take at least like 3 square kilometers of moderately sparse houses. Let's say you can stuff Goliath doll with 3 additional tons of C4 and still move it, it'd be about 4 tons of TNT equivalent, which is about as powerful as FAB-5000. That gives us ~200 meters of blast wave, and not much shrapnel (I doubt anything will be left of Goliath after that at all), so like a half of population if you time it during some village festival or something, not critical by itself.
Now thermobaric explosion would work nicely, but I doubt it's possible to sneak in hell of blood pools and get enough petrol to the surface without someone noticing.
>>1520344
> Nazrin and Wriggle causing a plague could do that
While devastating, plague wouldn't lead to complete extinction, not mentioning that Reisen is already selling medicine in the village and if needed I'm sure Eirin wouldn't mind making more antibiotics.
> Aki sisters ruining the harvest season could starve them
Also likely not fatal, surely villagers have barns for such cases so it would take at least few years to properly starve them, which is plenty of time for retribution. There are relatively wild forests as well where they could hunt in hungriest of moments as well.
> Medicine could probably do something horrible
She really depends on what we think is her limit, but if we suppose she can cast phosgene on entire village then yeah, easy win.
> Or Nitori could just build something in a shed for a few years and pull off a Killdozer type scenario to destroy the barrier.
Not sure if the barrier is a physical entity...
>>1520517
Good suggestion, and might work well. One second + one oni punch per person and entire village dies in 15 minutes. Though we know that zashiki-warashi spy on each household so such plan will be reported immediately. Wether they'll react immediately and be able to stop killing spree though is ambiguous.
>>1522156
Fail, youkai don't consume as much resources as gods or humans. It's also a possibility Reimu will murder you right away for same reason she murdered fortune teller - conscious pumping of youkai numbers, disrupting balance yada yada.