I did a little bit of editing, but I didn't change the plot at all. I think it reads better this way. Let me know what you think!
Korra leapt into the air, creating a dust cloud where she stood moments ago. "That was too close," she thought to herself before doing a back flip onto the cliff ledge. "What's the matter, Onigumo? I thought you were going to turn up the heat-" She hardly had time to dodge before another fireball flew past her.
"Shut it, wench! My flames will devour you!" Onigumo retorted. He never expected the avatar to fall into his trap so easily. She hadn't even noticed their location. They were near her home village, and he needed to finish her off fast if he didn't want to die of exhaustion. Little did he know that Korra actually had noticed they were close to her village.
"I wonder what he's scheming…" she wondered. She sent two boulders hurling towards Onigumo, but they disintegrated, never standing a chance against his unrelenting flames. "I should have foreseen this with the coming of Sozin's Comet! Why didn't I prepare?!" Korra was well into her fifties, and although her bending techniques had become stronger and more refined, she herself was feeling the exhaustion that accompanied aging. "What exactly are you planning to do, Onigumo?!"
Onigumo stopped. He smiled slightly before turning towards her with a crazy look in his eye. He knew the only way to kill the avatar forever was to slay Korra while in the avatar state, and he understood that meant getting on a personal level with her. The avatar didn't just transform into the avatar state for every battle; he was going to have to do something drastic. Without breaking eye contact, he replied, "My plan is simple; I'm going to destroy the avatar…permanently!" He flew past Korra, and before she could react he set her village ablaze.
Korra couldn't believe what she had witnessed. She just watched her whole village burn to the ground with everyone inside—and she couldn't do anything. Her body went numb, and her chest started to ache with the loss of her friends and family. A scream pierced the air; she didn't know whether the scream was from a dying villager or from some lucky soul who'd been fortunate enough to escape from the fiery death. She realized the terrible scream was coming from her.
She felt an immense energy surge through her body. The power from all the avatars before her was at her fingertips. All she could think of were the countless lives that were just sacrificed while she was little more than a spectator. To Korra, Onigumo was nothing more than a heartless murderer. Her thoughts raced through her mind. "How could he do this to innocent people?! Onigumo is a demon! He doesn't deserve to live!"
Onigumo flinched at Korra's cries of pain. He'd never heard something so frightening. It was as if she'd pierced his very soul. He felt a change in the winds, and watched with pleasure as Korra began transforming into the avatar state. All he had to do was wait for the opportune moment.
"There!" Onigumo spotted an opening in Korra's defenses in the middle of her transformation. He hadn't changed stance since he blasted the village, and the entire time he'd been concentrating all his energy into a single, powerful lightning bolt. Time almost seemed to stop. He took his aim and prepared himself to strike. "If this be my last bolt, strike true!"
Korra turned to face Onigumo with her ghostly pale eyes to deal the fatal blow. She cursed, "Let my spirit not rest until justice has been done in the name of all the poor souls that have suffered by his hand!" A flash of white light bleached the landscape, and two limp figures were all that remained in the prevailing darkness.