A fight scene must be immediate and hard-hitting. Whatever you do, don’t interrupt the scene for flashbacks, information dumps, long descriptions, or introspection.
Think of your fight as a scene in a movie. The camera angles constantly change (too much in modern films ’cause they can’t choreograph a fight worth beans in HollyWeird any more). People are swinging at each other and grunting as they get hit. Chairs and other furniture are destroyed as fighters fall on or use them as weapons. In books, you can even smell things if it’s important. The Point Of View (POV) character’s entire world is filled with danger, tactics (if s/he has time to think), ducking, jumping, pain, blood, adrenaline, loud sounds, the stench of the other guy’s armpit if he’s in a headlock, the smell of vomit, the taste of his own blood, the sting of sweat rolling into his eye along with blurred vision, the feeling of his fist hitting the other guy in the gut or his pockmarked face.
Battles are pretty much the same thing only magnified. Cannons (or blasters) deafen the POV’s hearing. She fires at an enemy soldier, the recoil of her high-powered rifle elicits pain from a shoulder already bruised by too many shots. The guy next to her falls, his brains blown out by an enemy round. Fighting nausea, she looks up to see her Colonel standing in the midst of the hail of bullets, talking to aids who are yelling into walki-talkies. The smell of spent gunpowder fills her nostrils. To the left, she sees a squad of enemy soldiers trying to flank her position, so she yells to one of her corporals to intercept them with his squad. An enemy tank rears up on the ridge just in front of her, but Johnson fires a LAWS rocket into it’s belly and it explodes, showering shrapnel over everyone in the area (she ducks and some of the shrapnel bounces off her helmet with a sharp “snicking” noise).
The previous paragraph would make a good fight scene if it were reworded, expanded, and actual (short) thoughts and dialogue inserted.
Bar fights and battles are all more real and hard-hitting if told from the viewpoint of a participant. Give the reader sights, smells, sound, and tactile feelings and your fight will be more real and harder to put down. Make the reader feel all of this and you’ve got a dynamite scene.