This is how Defeating the Tantabus goes in My Little Pony Transformers Prime: Do Princes Dream of Magic Sheep.
[The ponies in the dream world keep trying to defeat the Tantabus]
Thunder Streak: I cannot hold this dream together much longer! Equestria will fall because of me!
[The Tantabus feeds on his guilt]
Spike: Am I crazy or is that thing feeding on his guilt?
Twilight Sparkle: It IS feeding on his guilt.
Princess Luna: Wait. [realizes something] That thing is feeding on his guilt because he hasn't forgiven himself for what he did as Thunderwing yet.
Thunder Streak: It's true. I haven't.
[The Tantabus continues its feeding]
Twilight Sparkle: But why?
Princess Luna: And how?
Thunder Streak: I created the Tantabus to make my dreams into nightmares to punish myself for what I did as Thunderwing.
[The Tantabus finishes its feeding and takes on the form of a giant Thunderwing]
Twilight Sparkle: Whoa.
Princess Luna: Thunder Streak, you must forgive yourself.
Thunder Streak: I don't think I can.
[The Tantabus uses its fingers to open a portal to the real world]
Twilight Sparkle: You can.
Princess Luna: You must.
Thunder Streak: I don't know how.
[The Tantabus starts making its way into the real world]
Twilight Sparkle: Come on.
Princess Luna: You have to.
Twilight Sparkle: Thunderwing would've wanted the Tantabus to turn Equestria into a nightmare! You're doing everything you can to stop it! Don't you see? That proves you're not the same you were then! Everypony who knows you knows that Thunderwing is in the past! We all trust you, Thunder Streak! Do you trust us enough to believe we're right?
Princess Luna: Listen to Twilight, Thunder. Forgive yourself.
[Thunder Streak finally forgives himself]
Thunder Streak: Thank you.
[The Tantabus is erased from existence]