I have an idea for a dialogue scene that would motivate Fluttershy to fight Pinkie for what happened.
In a forest area in Ponyville, all alone, Fluttershy was crying her eyes out, once again from Pinkie insulting her, pressuring her and not being subtle at all. And not to mention still bragging on the turkey call champion when Big Mac lost his voice.
Fluttershy looked up and saw Party Thrill looking at her in concern.
Fluttershy: Oh, Party Thrill, it's you!
Party Thrill: What's wrong, Fluttershy?
Fluttershy: (sniffles) It's... Pinkie...
Party Thrill: (suddenly frowns, holds up her hoof) Stop right there, I think I get it now. You were working with the animals, singing to yourself, and your friends were not only there, but so were those who never heard you sing beautifully before; the Cutie Mark Crusaders, Discord, Jakk Checkers, Colby Lollygagger, Matelda, Tough Apple, Feathermay, Firecracker Burst, Roseluck, Cherry Pie, Plumsweet and Flitterheart, and after commenting about Fluttershy's voice, Pinkie criticized you in an unsubtle and insulting manner that made you cry.
Fluttershy: (cries) Yes!
Party Thrill: Fluttershy, are you just gonna keep on letting her do this to you now?
Fluttershy: (sniffles) No, but I can't do anything about it...
Party Thrill: Don't you remember when you are always friends?
Fluttershy: Well... yes.
Party Thrill: And friends don't make each other cry at all. It's just plain mean! You don't want her to get away with that, do you?
Fluttershy: Well... no.
Party Thrill: And I know you're not satisfied with his class, but remember one of Iron Will's rules?
Fluttershy: (getting angry and determined) "Treat me like a pushover, and you'll get the once over."
Party Thrill: That's the spirit! Give that to Pinkie and teach her a lesson for that problem!
Fluttershy: Right! I will!
Party Thrill: (points back to where Pinkie and the others were) Now get in there and fight!
Fluttershy narrowed her eyes as she gets in a fighting position.
Party Thrill: Fight!!
Fluttershy gritted her teeth in rage as she snorted.
Party Thrill: FIGHT!!!!
Fluttershy growled like a beast before zoomed off at really high speed.
Party Thrill: (as she waves a checkered flag) GO, FLUTTERSHY, GO!!!
Fluttershy flew like a comet as she headed towards Ponyville, ready to face Pinkie in what was about to be another One Chapter Beat Down.