Once you have figured out what the characters have to lose in a fight scene, you must decide on each character's special abilities, weapons, etc.
Your viewpoint character/hero's special abilities, weapons, and skills should have been set up long before this fight scene so it won't look like you pulled new abilities or weapons out of the air for your own convenience.
List the special abilities of the viewpoint character then give his opponent a skill or weapon that is equal to or slightly better than his. Equal powers make interesting contests. Extremely unequal powers make for a dull fight.
Now, you can map out the coming fight. Remember that the hero must barely survive each kind of attack, and he must start running out of options.
Especially in the final showdown, the hero must be forced to go beyond his abilities and must face some element of his ultimate fear. He must do what he considers unthinkable or impossible to win.
In an unpublished novel, I had a hero who must face a were-dragon. This was the climatic fight between the two characters, winner take everything. The hero, who wants to die because his life will be a living hell, must survive for the sake of the woman he loves because her life is at stake, too.
I wanted him to face his weakness and fear of living as well as his own tendency to care more about himself than anyone else.
Since this is the climax of the novel, I wanted the fight to extend over several chapters, and I didn't want it to be boring and repetitive.
First, I thought about the weapons of a dragon -- claws, teeth, fire, size, and wings. Considering the dragon's many weapons and ways to fight, I realized that I could divide the fight into three acts.
The first act is ground-fought and will involve fire. The dragon will also use its human intelligence and voice as an emotional weapon.
The hero is tentative in his skill, and he's distanced himself from fights before so his weapon is a lance. He has a magical shield and armor which will help against the flame, but he can't survive the flame for long, and the dragon is creating a conflagration with the vegetation.
The hero's uncertainty is also used against him by the dragon with its taunts until the hero acknowledges his feelings for his lover, and this allows her to bring magical rain.
In the second act, the dragon has lost its fire because of the heavy downpour which has soaked the terrain as well as dousing its flame so it takes flight, and the two battle.
I thought about flying warfare and the different ways a dragon can use its weapons in flight. I decided that the dragon would strafe the hero by using its claws to attack, and its wind in flight would be so strong the hero could barely stand to face it. The dragon would also use its weight to knock the hero down.
After the initial fighting where the dragon uses these methods of attack, it manages to get the hero's shield which he's used against the claws and proceeds to shred him at each pass and exhaust him because of the heavy wind created by its wings.
Barely staying on his feet because of exhaustion and blood loss, the hero finally retaliates by using the lance like a spear and throws it into the dragon's underbelly.
In the third act, the dragon can no longer fly because of damaged wings from the lance so he and the hero are forced to face each other in close quarters with no retreat. The hero uses a sword.
The hero now knows his own heart and has discovered his courage. He will no longer give up the fight. The dragon has discovered that it can die in this fight, and it's afraid for the first time, but it's forced to stay because the two are locked in a mythic pattern which neither can escape.
Since the battle is in close quarters, I thought about the dragon's different weapons, and the hero's battle plan. The hero must get close enough to stab into the dragon's heart, but the dragon uses its long neck, its size, and its speed to stay safe. The hero finally uses a distraction to shift the dragon's attention and stabs him.
Notice how I negated the different weapons of the dragon so that the fight itself changed dramatically, and the hero was forced to use different weapons, both physical and emotional, against each change.
The battle is also hard fought on both sides, and the hero wins more from sheer cussedness than any skill or weapon.