Don't focus on characters. Write your story like you're playing with action figures. Put in whatever cool stuff you want. You can include a connecting thread between the cool stuff but you don't really need to. While you will be writing a lot of fight scenes, you don't need to bother doing any research on tactics, strategy, martial arts, period combat, weapons, armor, or any of that. Fights should be won on vague concepts like "strength" and "power" and "will". It's great to have your characters go on long monologues about the importance of these concepts but you should never, ever define them, establish what their limits are, or state what's entailed in developing them.
Remember that your main character shouldn't work too hard for anything. He or she should just have the universe and other characters bend to suit them. It's ok if your character is directionless and is pulled through the story without motivation. Having your characters drive the story means you may not encounter every action set-piece you want which isn't acceptable. Not to mention emotional stakes will just bog down the fight scenes.