I wrote a four-part series over at Go Teen Writers on how to write fight scenes and battles. Here are the links:
My first post, Writing the Action/Fight Scene, talks about three things you need to know before writing out your fight scene.
In Editing the Action Scene: Seven Ways to Make it Stronger I talk about believability, research, cliches, dialogue, pacing, and what happens after the fight.
In The Wizards Duel I talk about what makes magical battles different from other fight scenes.
And in The Great War I discuss how to tackle writing out a major war in your novel.
Have any questions? Leave a comment here or on the Go Teen Writer blog.