Here's how Brian and Sunset Shimmer join the fight in Revenge of the Giant Chicken.
[we sideswipe back to Kamino where we see the battle is still raging on, and we aslo see that Barret has engaged Celestia]
Barret Barricade: Celly, stop! You're pregnant remember?
Princess Celestia (darker voice): Then she can be my little inquisitor!
Barret Barricade: Then what about Sharon?! How do you think she'll react to find out her Mother and younger sister turned into the very things we vowed to destroy!?
Princess Celestia (darker voice): Deal with it! [force pushes him back]
Barret Barficade: NO, I WILL NOT!! I'M NOT GONNA LET THE ONE MARE THAT GAVE A REASON TO LIVE, BE CONSUMED BY A PECE OF POULTY!!
[they firously clash sabers, as the other continues battling the clones]
[Soon, Brian, Sunset, and K2 arrive]
Sunset Shimmer: [hands K-2 a blaster pistol] You'll need this.
K-2SO: My goodness! Shut me down!
[then Hiro's shuttle lands]
Princess Luna: Surprise!
Brian: Luna, Hiro!
Hiro: You really think we wouldn't join in this battle. In a situation like this, it involves every single member of the Republic.
Princess Luna: Now, let's destroy these clones!
Brian: Right! [ignites his Lightsaber]
[They charge strait forward as it cuts back to Coruscant]