Say you're writing a Hero's Journey story. Your Hero for simplicity sake is up against the Dark Lord. You know the climax of your book is the Hero summoning the Great Dragon, to help defeat the Dark Lord in one on one combat (the Dark Lord's mounted another dragon).

From this scene alone (this is the only scene your working with). How do you choose the tone of your story, and work up to this climatic moment, from the beginning of the story?

However, what I'm trying to achieve (not this specific climax) is a story where the Hero isn't the 'chosen one, ' you know... the type of story where anything is possible just because it is.

I got a climax in mind that's radical in proportion, but my characters are just guys on a journey. One guys is smarter than the others, but doesn't have all the answers. How do I build up to the climax realistically without Gandalf turning up to help them?