This is how Celestia vs Twilight goes in Sideswipe's Sirenix.
[Celestia stands near some doors]
Princess Celestia: Go on. I'm going to find Twilight.
[The engines fire up]
Strongarm: [to Celestia] The blast doors to the command center are locked!
[Celestia looks down as the ship leaves]
[Princess Celestia approaches a cloud of smoke]
Princess Celestia: Twilight?
[Twilight, the Element of Magic still on her head, emerges from the smoke]
Twilight Sparkle: I've been waiting.
Princess Celestia: Twilight.
Twilight Sparkle: What... took... you so long?
[Twilight strains and grasps her head in pain]
Twilight Sparkle: What did he DO to me?
[Flashback to Umarak turning Twilight evil]
Umarak the Hunter: And some lingering remnants of her memory and personality. But without my acknowledgement, she would be utterly mindless. An ordinary Alicorn! An ordinary Alicorn...
[End of flashback]
[Twilight continues to strain. Celestia approaches and helps Twilight]
Princess Celestia: You're very sick. Let me help.
Twilight Sparkle: I... have... to--- [fires at Celestia, throwing her back]
Princess Celestia: I don't want to fight you.
Twilight Sparkle: Then... stop me!
Princess Celestia: But how? What should I do?
Twilight Sparkle: Please Celestia!
[Twilight blasts Celestia with magic and she fires back. Smoke fills the room. Twilight strains again]
Twilight Sparkle: No... no!
[Celestia shoots her]
Princess Celestia: This is no time for games Twilight!
Twilight Sparkle: Not... a game! Element of Magic makes me too strong!
[Twilight gets up]
Princess Celestia: That I can see.
[Twilight pins her down]
Twilight Sparkle: If you lose -- Autobots.... all... destroyed!
[Celestia crinches in pain]
Princess Celestia: Thanks for the tip!
[Celestia tosses Twilight across the room and Twilight hits a wall. Celestia approaches Twilight]
Princess Celestia: Twilight, we can save you.
[Twilight groans painfully and blasts Celestia with magic. Celestia hits a wall. Twilight approaches]
Twilight Sparkle: You... should... have... finished me.
Princess Celestia: If I did, the Magic of Friendship would be lost forever.
[Twilight prepares to fire. Celestia slumps to the floor. The Element of Magic flares, successfully overriding Umarak's dark energy]
Twilight Sparkle: Until... all... are.... one!
[She removes the dark orb]
Twilight Sparkle: A monster. He made me... a weapon... to destroy the very ones... I loved in life.
[Twilight places it on Celestia's horn]
Twilight Sparkle: But you can still save them, Princess Celestia.
[Princess Celestia groans in pain]
[Princess Luna arrives]
Princess Luna: Tia.
Princess Luna: Are you alright?
Princess Celestia: I think so.
Princess Luna: Let's hurry.
[She helps her up]
[Twilight looks at Umarak's fast approaching ship]
Twilight Sparkle: Princess Twilight Sparkle to Autobot fleet. Return to Canterlot. That is my final command.
[The Autobot fleet move to Canterlot]
Umarak the Hunter: The Autobot fleet is escaping! And their flagship is on a collision trajectory for the detonator! Direct all firepower on the flagship. Obliterate it!
[Umarak does his worst, to no avail; Twilight remains in her seat even as the explosions happen]
[Umarak looks shocked]
Umarak the Hunter: It's not stopping! No Autobot or pony could survive that.
[Twilight stares on]
Twilight Sparkle: Until all... are.... one...
[The flagship arrives]
[The Autobots arrive]
[Luna and Celestia appear]
[Sideswipe hugs Twilight]
[Twilight smiles at him]
Sideswipe: You okay?
Twilight Sparkle: Yeah.
Umarak the Hunter: Until next time, I'm gone.
[He travels through shadow]
[Sideswipe holds a Winx pendant]
Sideswipe: The Winx gave it to me.
[He puts it on his neck]
Sideswipe: I promised them that I would never let it's light go out.
[Sideswipe stares at the other Autobots]
Sideswipe: We all must protect it.
[Sideswipe stars into deep space]
[The episode ends]