If you have never been in a life-or-death situation and you have a battle in your book you probably need a little help. Usually I suggest trying something that you're going to write. In this case, I don't suggest finding the closest war zone. Instead try a self-defense class. Interviewing veterans and police officers is also a great way to get information on what it actually feels like to be under fire. Books on military strategy and museums can give you ideas for the scene.
The reader needs to understand what the motivation of the attacker is. This can be a very concrete goal like getting an artifact, claiming a castle, destroying a bridge. If you are attacking something nonhuman, like evil demons your entire goal can be to exterminate the opposite people. Defenders have very similar goals in most circumstances. Defenders are always trying to stay alive, keep their possessions and retain their position.
Briefly describe the relevant features of the setting. Have your character assess the escape routes, hiding places and potential weapons in the area before the fighting starts. Tell us where the allies and enemies are in this space.
Instead of just telling us your characters’ state of mind, describe their bodily reactions. A pounding heartbeat and sweaty palms are common for a novice fighter. An experienced warrior might actually get really calm. He might ready his body for the attack with shallow breathing and intense concentration on his senses.
Show fears, worries, concerns, hopes, irritation. This is a life or death situation. Make it real, make it human. A fight is very emotional.
Use concise full language to describe the actions. A fight is largely made up of actions and consequences. Quick sentences happen fast. A really long sentence that meanders across the page like a trickling brook in the lush valley of the Shenandoah slows down the entire story.
Dialog slows down a fight scene. A character needs to be able to breathe and concentrate in order to fight. Keep dialogue short and necessary, unless you are using it for a humorous effect.
Intensify the scene by showing less description and dialog as the fight heats up. Show more actions, the bloody results, and some sensations. Pain is a great way to signal that this fight is real.
If the fight is really intense your character will stop noticing their body completely. This is called The Zone by athletes. They may also fail to notice their injuries after the battle is over. That is called shock.
The next day, have your character deal with injuries and the trauma of being attacked. If you don't show any impact the intensity of the scene is lessened. Nightmares, increased alertness, insomnia, abusing alcohol, there are lots of ways to show the scars of war.
Victory is always bittersweet. Warrior cultures always have specific ways to honor their fallen comrades. Invent your own, or do some research. War isn't a frickin' board game.
I admire the fight scenes and warrior culture create by J.R. Ward in the Black Dagger Brotherhood. Who wrote your favorite warrior?