This is how Princess Luna vs. Nightmare Sparkle goes in My Little Pony: The Revenge of Nightmare Moon.
[Nightmare Sparkle goes over to Luna]
Nightmare Sparkle: Luna, Luna, Luna. You had it all. How foolish you were to give up your power! And for what - these weaklings? Ha! If you had taken my offer and became Nightmare Moon once more, your precious ponies would be spared. You've done this to them, Luna, and they will never forgive you.
Princess Luna: I've forgiven myself for what I did back then. And now, I will free Princess Twilight from your dark control and rid Equestria of your eternal darkness!
Nightmare Sparkle: Twilight Sparkle was weak. I destroyed her.
Princess Luna: Twilight herself may have been weak but her friendship was strong.
Nightmare Sparkle: I have brought peace, freedom, justice, and security to my new Nightmare Empire.
Princess Luna: What?!
Nightmare Sparkle: Today is the end of the day. My power will bring an end to the light, to the daytime. All ponies will bow to the Nightmare Princess! And will remember this as the last day of the daytime!
Princess Luna: I will not let you reach the Tree of Harmony!
Nightmare Sparkle: I WILL reach it and corrupt it with Nightmare!
[Princess Luna fires a beam of magic at her, but Nightmare Sparkle dodges it]
Starlight Glimmer: Sunburst, you know a spell that can reverse this! Use it!
[Before Sunburst could use the spell, Nightmare Sparkle lifts him up with her magic and throws him to a tree, knocking him out. Nightmare Sparkle then looks at Luna]
Nightmare Sparkle: TRAITOR!!!!
[Princess Luna fires another beam of magic but Nightmare Sparkle reflects it with her wing, which it bounces off and hits the ceiling, creating a hole in it]
Nightmare Sparkle: [smirks] Thanks for opening the front door for me, princess!