Nightmare Moon attacked first. Her horn fizzled with blue magic for a split second, then she disappeared, leaving powerful magical residue. Daybreaker wasn't expecting a teleportation right from the get-go, so Nightmare managed to land a small beam at Daybreaker, pushing her back a little.
Nightmare then teleported a second time, shooting again. This time, Daybreaker was ready, and she countered with a quick beam of her own. The point where the two beams collided caused a mini explosion, smoke trailing along.
Now it was Daybreaker's turn. She flew high up into the air, and yelled, "Time for payback! Here is my strongest attack! Solar Hurricane!!!" Nightmare opened her eyes wide and formed a protective barrier around herself, slowly increasing it's durability until it was virtually indestructible. While Nightmare did this, Daybreaker was charging up her attack, and she maliciously grinned once she had focused the necessary energy. "Get ready to meet your end!" Daybreaker shouted with certainty. Then, she extended her wings to their full length, and roared a battle cry.
Nightmare closed her eyes, bracing herself for just in case the shield she had materialized was penetrated. Unfortunately for her, Nightmare's fears came true. Daybreaker's body became engulfed in a thick, dense layer of pure, solar magic. Then, all the energy climbed up to her head, and she repeated a technique she'd used earlier, except this was much more powerful. She opened her mouth and a huge column of flames gushed out of it, launching itself at Nightmare.
This blast was the size of a hurricane, hence it's name, and it instantly blew away Nightmare's shield, causing her to gasp as the blast of flames leapt onto her. "AAAHHHH!!!!!" Nightmare wailed, an inferno covering and burning her from head to hoof.
Daybreaker burst into a fit of laughter at this, while still barely managing to keep her focus and discharging a steady flow of scorching hot fire.
Nightmare realized she was in a very bad position. Obviously. But then she remembered something, and she would have slapped her forehead with her hoof if it wasn't being burned and unable to move from the intense pressure of the heat. She tried to focus her magic, which took longer than usual, then she used a simple teleportation spell to get out of her most powerful enemy's greatest attack. Once out, she drew attention to herself, startling Daybreaker and forcing her to release the technique.
"Ahahaha! You can't defeat me with that. In fact, any skilled unicorn could get out of that with a mere teleportation," Nightmare noted. Daybreaker cursed at her careless mistake. Nightmare saw this, then boasted, "My Nightmare Fury is far superior to your puny Solar Hurricane. Though I admit your attack may deal more damage, it has a fatal weakness, unlike my spell. It seems night will always be greater than day!" With that, she rapidly flew towards Daybreaker and they engaged in a back and forth clash of magical lasers.
After the thirtieth collision, they both slowly landed on the ground, panting and gasping from exhaustion. "Heh. Heh, heh, heh! HAHAHAHAHA!!!" It seems my plan succeeded," Nightmare declared triumphantly.
Daybreaker looked confused, and she asked, "What plan do you speak of?"
Nightmare smiled wide, then replied, "I have you tired enough that one more heavy wound will finish you! Once again, I unleash...Nightmare Fury!!!" Daybreaker's eyes instantly shot up at this.
"Damn it! I've been a fool! I must stop her!" she said to herself, then she shot at Nightmare, but just before she could blast a ray of heavy magic at her, Nightmare finished charging up, and she once again plunged the world into total darkness. Daybreaker cursed loudly enough for Nightmare to hear from miles away, and she laughed in answer. Daybreaker again was engulfed in the whirlpool of black energy, and this time, she knew no amount of begging would save her.
Daybreaker writhed and thrashed around, trying everything she could to get away and protect herself. This effort was futile though, the expanse of black magic was too huge, and any shield she enveloped herself in got melted away instantly by the power of the night. Daybreaker resigned herself to her fate, knowing anything she did would just waste energy, and instead decided to just wait for Nightmare to run out of energy.
"...Aha!" Daybreaker said, thinking of something that may work. "Maybe an attack as great as my Solar Hurricane can defeat this spell!" So, she drew a massive amount of power into her throat, and pushed it up until her fireproof mouth was brimming with godly flames.
She went insane on the ocean of dark energy. Daybreaker's pent up ball of super fire gushed out, and she span around, blasting everything within thousands of meters of her to nothing, including the gigantic ocean of Nightmare's magic. The flame remainders stayed where they were, miraculously not vanishing into thin air like everything else not controlled by Nightmare here. Daybreaker spewed out more and more, the fire bending to her will and surrounding her in a tornado of intense heat, partly a shield, partly a quick way of travel, partly a means of emergency offense, and all awesome.
Daybreaker's harsh, black and yellow eyes blazed with fury and determination. She closed her eyes for a second, concentrating deeply to sense the strongest magic in the area, and once she found it, she willed the fiery tornado swirling around her to rush over to the strong signal.
Strangely (and amazingly) enough, the twister of flames made it possible for Daybreaker to see, and the cyclone was clearly visible to her. This made it so she saw Nightmare once she got close, and the night princess was, needless to say, very surprised to see Daybreaker.
"What, how!?" Nightmare asked with disbelief. "That pool of black magic should've been impossible to break out of!" Daybreaker laughed at Nightmare's incredulity, then charged her. Nightmare, though, still had the upper hand, because, as you may remember, Daybreaker was at half strength and Nightmare was 10x stronger than usual.
Daybreaker first flew at Nightmare, spinning around to increase momentum, and turning the vortex of flames into a barrier so compact, there weren't any gaps. Nightmare got out of the way, but Daybreaker went up and around, shooting at Nightmare once again.
Realizing this wasn't going to work, Nightmare focused energy into her horn, and slowly, a dark glow enshrouded her, and she then charged at the quickly approaching Daybreaker.
When the two super powerful auras clashed, a mixture of black and red flashed between the princesses, and it started to become hard to keep their attacks going, as the pressure from the point of collision was too high.
Finally, after half a minute, it was too much for Daybreaker to handle, and her protective tornado of fire dispersed, then vanished into thin air, and the strengthened princess of the night bashed her dark attack into Daybreaker's stomach, making her gasp and crash onto the nightmare floor, once again blind without her super flames.
Fortunately for her, the maximum duration Nightmare could use the spell for had reached it's end, and her spell switched off on it's own.
Both of them were out of breath and slightly injured, but they weren't ready to give up just yet. Daybreaker panted and her horn glowed red. Nightmare mirrored this, except with her horn glowing dark blue, and they faced each other, aware this may be the final clash.
Just before they fired, a loud, commanding voice shouted, "ENOUGH!!!" A flash of bright yellow blinded the fighters, and then, Celestia appeared. She brought her wings high up, her eyes glowed pure white, a powerful aura momentarily gleamed around her long, majestic horn, then she fired two beams at once. Nightmare and Daybreaker countered this with the lasers they had been charging up, but Celestia blasted through them both, and the evil princesses vanished simultaneously.
Immediately following this extensive use of her already drained magic, she collapsed onto the ground, and breathed heavily. "I-I did it," Celestia said, a smile slowly creeping onto her mouth. Then, she looked at Starlight still trapped in the bubble, and she released the spell, letting Starlight out. She grinned, and congratulated Celestia on her victory over both of the evil princesses.
Then, they saw Luna, lying on the floor, and they used magic to restore her energy, which had been sucked from her by Nightmare earlier.
Luna woke up, and she spun her head around, looking for any threats. Then, when she found none, she too smiled. "You did it!" She cried, looking at Celestia. Then, she gave her sister and Starlight a curious look, and asked, "All I remember is Nightmare Moon taking my magic, then everything went black. Can anyone fill me in on what happened after that?" So, Starlight talked about everything that had happened after Luna had fell unconscious. Luna pondered something for a while, then asked anther question. "Wait, if Daybreaker was already stronger than Nightmare Moon, and stole my sister's energy, which is stronger than mine, why were the two equal?"
Both Starlight and Celestia thought about that for a while, then Celestia finally came up with an answer. "I have to assume it's because I was already low on energy and not conscious when my magic was being drained, while you were awake and had full energy." Luna nodded at this answer, and the three proceeded to exit the dream world.
There, Celestia and Luna told Starlight her mission was complete, and of course, Twilight came interrupting and dragged Starlight to ask questions, while the two princesses had a friendly sister conversation.