This is how Sadness' idea and the answer to stopping Yveltal goes in Thomas and Twilight Go To The Hero Factory.
[The next morning, Thomas awakens, tears still in his eyes]
Joy: You still feeling bad about what happened last night, Thomas?
Anger: Finally, morning's here.
Sadness: Thomas, are you ready to tell us about Von Nebula's past now that you had some time to think about it?
Disgust: This had better be good.
Fear: Oh, I hope there's no scary parts to the story.
Sadness: There isn't.
Fear: [sighs] Good.
Joy: Okay, Thomas. Tell us Von Nebula's past.
Thomas: Okay. It all started like this. Before we even came to Hero Factory.
[4 hours later....]
Thomas: And then Von Ness went rogue and violent, changed her name to Von Nebula and has been bent on revenge on Stormer ever since then.
Anger: I cannot believe someone like Von Nebula would leave her teammate and leader behind like that.
Thomas: I know.
Disgust: It's like Von never really understood Stormer like we do.
James: Is there a way to turn her back into Von Ness again.
Zane: P.I.X.E.L, is there a way to make Von Nebula who she once was again?
P.I.X.E.L.: Yes, Zane. But the only way to do is try and remind her of the good things she did as Von Ness in the past.
Zane: I see.
Thomas: Did you get everything P.I.X.E.L said, guys.
Anger: But we need to find a way to break these chains and get out of here. Jeez. Do these have to be so tight around your wrists?
Anger: I was talking to myself, idiot.
Thomas: Anyone have any ideas on how to break these chains and get out of here?
Anger: Come on. It's not like we can trick the Nindroids that are guarding us.
Joy: That's it!
Joy: We can trick them!
Disgust: But how?
Joy: Pinkie, any ideas?
Pinkie: Got one.
Cryptor: Remember the time when Rotor told that awesome joke.
Min-Droid: Yeah. I laughed so hard, oil came out of my nose.
Pinkie: Hey, fellas.
Pinkie: My emotion friend here wants to say something.
Sadness: I just wanted to tell you that Von Nebula wants to see you.
[He and Min-Droid leave]
Ssdness: [unlocks the door]
Anger: [clears throat] Chains first, remember?
Sadness: Oh, yeah. Sorry. [unlocks the chains around their wrists]
Brian: Ca'mon we got to stop Yveltal!
[We see Yveltal's destruction]
Anger: Wait a minuet! How did it just destroy itself?
Thomas: John, how did Yveltal destroy itself?
John: It must have happened because somebody, who is an emotion, used his or her ability to make it realise what it was doing wrong.
Brian: I'll do it.
Joy: Very well, Brian.
Anger: What are you gonna do?
Brian: Stop Yveltal. [walks up to him]
Sadness: Okay, Brian. But be careful.
Brian: Hey, you!
Yveltal: [gets his attention]
Thomas: Sadness, have you ever faced a Pokemon like this before?
Sadness: Yes. And I have my own. It's a Feraligator.
Brian: Come on!
Thomas: Sadness. You need to use you Feraligator.
Sadness: Okay. But, what if Brian gets hurt?
Thomas: Then you'll just have to use it to bring him back here.
Sadness: Okay. Come on out. [throws the Poke-Ball and the Feraligator comes out]
Thomas: Okay. Now we're ready.
Anger: John, any updates on that anti-virus yet, pal?
John: Not yet, Anger. Breez and the rest of the gang are getting the final ingrediant right now.
Anger: Well, tell them to hurry up, because we have quite a problem here.