Humanoid monsters? Humans would survive common orcs and hobgoblins just like the Gauls did living next to the Visigoths and Goths, etc. They would do so by being competent warriors and by being led by competent warriors or they would survive like the Romans did by employing competent warriors.
How about small predators? Stirges, dire rats, wolves, etc. Humans would survive their depradations in much the same way that they survived the real world depradations of wolves, lions, etc. Trained animals, decent weapons (slings and staffs are apparently enough to keep wolves from attacking shepherds too often), and regular hunts.
Large predators? Dire lions, etc. Real world people survive lions, tigers, and jaguars, so I imagine that fantasy humans would find a way. Much like real life, they would probably be more prevalent at the outskirts of civilization and people would probably be different at the outskirts of civilization too. Real life tribesmen from the amazon jungles have an entirely different set of skills from Boston accountants or even Iowa farmers. And, similarly, the typical 19th century frontiersmen might well have had different classes and more hit dice than the typical 19th century city dweller.
Unstoppable intelligent predators: Dragons, etc. It's likely that humans would survive such scourges the way they did in stories: sacrifice to them and pay them off. A princess for the dragon. Seven virgins each year for the minotaur. Etc.
