Starshine flinched. Twilight saw it, even as her attention was split between that and Nightmare Moon. She focused on the friendly alicorn enough to see her image wane. "I'm sorry," Starshine said, looking at her. "Sunburst needs me." With those words, she faded away. Twilight was left alone to stand against Nightmare Moon, right as the latter shattered her shield and growled at her.
She could do it. It wouldn't be easy, but she could do it. She had to. Nightmare Moon fired at her, but she was expecting it and rolled to the side through the air. She didn't stop there, rather she let herself fall and circled around Nightmare Moon, beneath her and to the side, to then push herself upwards again behind her. Nightmare Moon turned, ready strike again, but Twilight was faster. Conventional magic wouldn't do much, she knew that, but physics clearly weren't going to fail her yet.
A loud boom burst forth from her horn, a wave of compressed air suddenly released like an explosion. Had she not had the foresight to protect her ears with magic, she would have gone deaf from it. Judging by the streaks of ichor leaking from the sides of her skull, Nightmare Moon hadn't been as prepared. It didn't matter, she'd get better quickly. What did matter was that the burst had pushed her just a little farther back, and momentarily knocked her off balance.
Twilight wasted no time in getting to action, ignoring the weight she felt against her breast after the blast had compressed her own ribcage. Though she knew it was extremely risky, and potentially unstable, she teleported on top of Nightmare Moon. She did so quickly, before the alicorn had time to right herself in the air. Another blast shot out of her horn. Not magic, not exactly, but energy. A spell she'd been working on, one she'd never fully completed but one that was good enough for what she needed it for at that moment. It hit the other alicorn and pushed her, a burst of sheer motion, kinetic energy directly transferred onto what it hit without a magical formation to carry it. Unstable, unfit to be used for precise work, but perfect for hurling someone around if their well being was of no concern.
Nightmare Moon was struck in full, and in combination with her present lack of balance she was sent violently towards the ground. She fired a spell at Twilight, already having started to do so as soon as she'd sensed her teleportation, but her aim was off and Twilight managed to dodge out of the way.
But it was not the ground that Twilight had meant to send Nightmare Moon towards. Indeed, the direction was the same. It was what was between the ground and her. With all the speed the spell had imprinted on her, Nightmare Moon was sent straight down towards the portal still open in the air above the Empire. Twilight subconsciously bit her own teeth in anticipation during the few fractions of a second it took for the distance to be covered.
But Nightmare Moon realised what was happening too quickly. She couldn't right her position, she didn't have time to, but she didn't need to. With a sound like that of a branch being violently broken from a tree, her body split itself in half, front and back, and each part fell past the portal on a different side.