Read other action scenes that you enjoy, and educate yourself on the techniques the author used. Pay special attention to the structure of the sentences, as well as the type of details and dialogue the author uses.
Figure out a place in which to hold your battle scene. Try to think of something that will create tension and a struggle for the characters, like a tunnel, precipice or cave. The setting will make the readers more intrigued by what occurs.
Outline and prepare for writing the scene by jotting down the particular events you want to happen. Keep track of who needs to do what, how they need to do it, and what the results will be.
Plan your scene using maps and diagrams. With the map, pinpoint places of action and document the areas to where your characters will travel throughout the scene. Consider drawing some pictures of the action you wish to portray. They will help you to describe the scene in better detail once you begin writing.
Determine what will change after the battle scene in terms of the characters and the plot. Be sure which characters, if any, you want to die, and which you want to survive, with or without injuries.
Write your scene in a rhythm that corresponds with the action. Instead of using slow, thoughtful language and syntax, write quickly and precisely. This means to use short sentences and even fragments to get your message across and to keep your reader intrigued. Phrases like "He leapt from the fire" and "She grabbed her sword, ramming it into the beast's stomach" are economical ways of keeping the pacing quick and making the scene more enjoyable for the reader.
Enjoy yourself while you write. Put on some exciting music, and get up and choreograph some of the fight moves when you feel like it. Let the scene carry you as you describe the action, and don't be afraid to take unplanned turns in your story to make the scene more interesting.