Well, writing good fight scenes goes hand in hand with writing good description. When I write fight scenes, I imagine what the characters would do in the given situation, say as in the case of a Kuroshitsuji fanfic I am writing, I have a fight scene between Sebastian and Grell . This is what a part of it looks like (Lilith is my OC btw):
"Sebast-chan!" the red haired grim reaper that Sebastian Michaelis loathed most was standing before him, over what looked like the corpse of young woman. The macabre image made him shiver although the tingling of his skin was barely detectable. "What's wrong? Why does my Sebas-chan look so morose? It does not become you at all!"
"I am not yours" Sebastian snarled, standing in front of Ciel protectively, the image of the woman's body was refusing to vacate his mind. As he looked at the cooling form of the dead woman he saw Lilith in her place, covered in blood, her ruby eyes cold and distant. The image made him gaze at Grell with cold hate.
"But…but…Sebas-chan!" Grell grinned, shark-teeth bared as he hefted his chainsaw, preparing to attack. "We could have so much fun together!"
"I highly doubt it" Sebastian drew a dagger from inside his coat, the tiny blade glowing with Demonic energy. It was one of the few possessions he had kept from his journey from Hell to Earth.
"Oh stop this!" Grell swung the chainsaw, the roaring blade missing the butler's throat by inches, even as he dodged the attack. "I hate seeing you so angry and upset! Tell Grelly-kun what's wrong!"
"It is of no importance" Sebastian replied coldly, his eyes glowing with annoyance as he swung the dagger at Grell, the Shinigami dodging effortlessly. As Grell swung the chainsaw again, Sebastian leapt high, landing on the blade itself and walking across it to leap behind the reaper who cried out in protest.
"This is no fun! You keep dodging everything! At least let me get a hit!" he hissed, his glasses slipping down the bridge of his nose. "And this is of importance to me Sebastian! You are important to me!"
"Which I find utterly disgusting" Sebastian growled back and Grell grinned wider, smiling with glee and the barest hint of lust.
"Oh, growl like that again!" the reaper exclaimed. "I love that sound almost as much as the blood upon the ground!" The Reaper's heart was racing as he spoke, adrenaline and lust powering his further attacks.
\"Never tickle a sleeping dragon...or else it may laugh so hard that it accidentally torches you with its fiery breath\"