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.
Next up: Writing a Character’s Physical Actions