This is how Princess Celestia and Princess Luna vs. Thunderwing goes in The Return of Thunderwing.
[Princess Celestia and Princess Luna arrive at Thunderwing's lair]
Princess Celestia: Thunderwing!
Princess Luna: We have come as you requested
Thunderwing: Princess Celestia and Princess Luna, I see you got my message.
Princess Celestia: Yes.
Princess Luna: Now hand over Twilight!
Thunderwing: Your attempts to get your beloved fourth Alicorn back are too late. Twilight Sparkle is now loyal to me.
Princess Celestia: What?!
Princess Luna: No!
Thunderwing: I've waited a long time for this, my dear Princesses of the Sun and Moon. At last, your fourth Alicorn is on Unicron's side.
Princess Celestia: Not if we have anything to say about it.
Princess Luna: You will fail, Thunderwing, one way or another.
[Princess Celestia and Princess Luna use their magic to blast Thunderwing]
Princess Celestia: Your rule is at an end. And it was not short enough.
Princess Luna: You are not going to win this fight. Not on our watch.
[Thunderwing gets back up, impressed at the blast Princess Celestia and Princess Luna gave him]
Princess Celestia: We will end this rule of yours.
Princess Luna: And you will fall.
[Princess Celestia and Princess Luna unfurl their wings]
Thunderwing: You two cannot stop me. With the Dark Spark, Twilight will become more powerful than either of us. [unfurls his wings]
Princess Celestia: Very well.
Princess Luna: We will fight you alone.
Thunderwing: So be it!
[They glare at each other for a moment, run at each other, and trade blows]
Thunderwing: If you two are so powerful, why have Twilight and her five former friends do your dirty work to defeat Discord and King Sombra instead of doing them yourselves?
Princess Celestia: Because they weren't friends at the time.
Princess Luna: So we couldn't task them with it.
Thunderwing: I see.
Princess Celestia: Well, now that they are, Discord has reformed.
Thunderwing: And what happened to King Sombra?