I came in to mention Blade Trinity, but not for the humans fighting vamps, but rather the ‘UberVamps’. Early in the movie Blade himself with the help of other vamps has massive problems in taking down one or two of the ubervamps, but by the end of the movie they’re cutting through them like so much wheat.
I’m certain I’ve read this in a Terry Pratchett book (based on your name, maybe you have too), although he might very well have borrowed it from Twain.
Books here seem to be echoing real life. The Matterhorn was seen as inaccessible at first, and one team experienced spectacular deaths trying to climb it. But by the 19th century, it was being regularly climbed by large parties of inexperienced tourists , many not even wearing climbing gear.
For that matter, look how many people have climbed Mt. Everest (and ignore the many deaths).
Could this be the Hundredth Monkey Effect, aka. synchronicity aka. morphic fields?
Or this earlier scene.