Ideally it would be something already in your world that *seems* so insignificant that the reader writes it off, but in the end turns out to be the key. You may already have something in the narrative that could work. Or, if not, you’d have to go back and add it in.
Of course, that is only one of many puzzle pieces you could mix+match to come up with a pleasing fight.
For instance, in the course of things you could allow the good guys to come up with the “perfect plan”, which seems to be working, until all of a sudden there is a turnaround — the badguys saw it coming and used it to their advantage.
And of course there’s the ever-wonderous”George McFly” moment: All seems lost, and MC is on the verge of giving up, but remembers the price of failure and with a newfound determination, rallies, fighting back with an inner strength that he never knew he had.
Homeschooling father of 10, writing Christian action/adventure novels from my home high in the Rockies.