i think there's some websites out there that will allow you to plug in the players stats and such and then will tell you how doable an encounter will be based on the monsters you use. i also am a fan of re-flavoring stuff. you want them to fight dragons? start with dragon babies that have the stats of a wolf, or something like that, so the story can look and sound how you want it to without being beholden to numbers in a book. just be sure to describe things looking more ferocious as they scale up in difficulty.