Here is how Chernabog gets defeated in Yuna and the Return of the Princess.
As the Righteous Armies prepared to fight.
Star Swirl the Bearded: (nodded at Sharon)
Princess Sharon: For Yuna.
Sharon runs forward towards the dark armies, The warriors of Equestria, And their allies charged. The battle is on as begin the fight, The Autobots and Maximals go into their robot modes.
On Mount Diablo.
The foals turns around and looking for Yuna.
We see Yuna running to the doorway, Smeagol and foals followed her.
Star Swirl and Derek continue to fight the Orcs, Goblins, Uruk-Hai and Demons. There is a screech from the Wyverns and Star Swirl stops to look up at one who is closing in on him. Star Swirl sees a moth flitting around him and looks up again to see Marahute intercept the fell beast and drag it off.
Princess Sharon: The eagles! The eagles are coming!
Hunter of Avalar: Led by Gwaihier and Marahute.
The eagles engage in the air with the Wyverns.
The foals looks and sees Yuna making the run to the door. She goes in. The foals follows Yuna and walks along the pathway into Mount Diablo.
Yuna is standing on the edge of the pathway looking down into the Cracks of Doom. She turns.
Princess Yuna: I'm here!
Snowdrop: Destroy it!
Yuna looks away. He holds the Amulet out in front of her on its chain, watching it.
Princess Skyla: Go on! Now! Throw it in the fire!
Yuna is struggling with herself. She stands looking at the Amulet. She looks down into the Cracks of Doom gasping and hesitating.
Nyx: What are you waiting for? Just let it go!
The Amulet seems to speak to Yuna. Yuna holds the Ring in front of her eyes. Her eyes look mad. The Amulet fills all her vision. She turns to look at her friends. She looks mad.
Princess Yuna: The Amulet is mine!
Armor Bride: No! Don't wear it!
Princess Skyla: (as Yuna puts on the Amulet) No!
Chernabog moves instantly to Mount Diablo, aware of him. The Wyverns turn away from the battle and speed towards the mountain.
We can see Yuna’s hoofsteps on the pathway. Sméagol appears and looks around. He sees Yuna’s hoofsteps. He jumps onto Yuna’s invisible shoulders. Yuna cries out.
Sharon is fighting on the battlefield, she hears a roar and turns around, a Cave troll is there and it attacks Sharon. Sharon fights the Cave troll desperately.
Sméagol is on Yuna’s shoulders struggling with him. Snowdrop tries to get up but cannot. Sméagol pulled the Amulet really hard. Yuna appears screaming. Sméagol steps away while Yuna get back on her hooves.
Sharon falls down and Celestia sees she is in trouble she tries to reach her.
Yuna was nauseous while getting back on her hooves. Sméagol holds the Amulet in his hands and looks horrified and moved back towards the brink.
Sharon turns around and the Cave troll is striding towards her. Celestia is desperately trying to reach Sharon to help her.
Sméagol: Sméagol must destroy the amulet.
The troll puts its foot on top of Sharon. She takes out her knife and stabs the foot. Star Swirl looks.
Yuna begins to grapple with Sméagol on the edge. They take a step too far and topple over. Sméagol grabbed onto Yuna's backlegs and dropped the Amulet into the Cracks of Doom. The Amulet sits upon the top of the lava.
Yuna is hanging on the edge of the rock, holding on with her good hoof. Snowdrop dives towards her. She tries to reach her and Sméagol.
Sweetie Heart: Yuna! Sméagol!
Snowdrop: Give me your hoof!
Yuna looks up at her. The Amulet begins to glow.
Snowdrop: Yuna! Sméagol! Grab my hoof!
Yuna tries to but slipped. But Sméagol climbed onto Yuna and on the edge.
Snowdrop: No! (Yuna swings back down. He looks desperately up at Snowdrop.) Don’t you let go! Don’t let go! Reeeaaach! (With one last desperate attempt Yuna swings her foreleg up and reaches for Snowdrop’s hoof. Sméagol grasps her other hoof. The Amulet sinks below the lava.)
Chernabog suddenly felt fires in his heart. It looks this way and that desperately. The Cave Troll stops, looks towards Chernabog, realises it is free and runs off.
Star Swirl turns towards Chernabog which is screeching and groaning with a terrible sound. Duck, Celestia and Sharon turn to look too. The mountain begins to collapse. Star Swirl has tears in his eyes. They watch as the mountain collapses to the ground. Chernabog explodes sending a huge shockwave outwards.
Hiccup: Yuna! Yuna!
Kaos laughs in Yuna's success. The Dark gate begins to collapse. Orcs, Goblins, Uruks and Demons try to run away, but the ground below them gives way. They are all destroyed. Mount Diablo erupts. Reggie looks shocked. Star Swirl begins to cry. The Wyverns fizzle and disappear. Celestia watches with concern. Sharon cries.
Princess Sharon: Yuna.
The foals and Sméagol run along the pathway leaving the mountain as it erupts around them. They escape and manage to jump a chasm onto a high rock, avoiding the lava flow which pursues them.
Yuna stands still, her eyes look clear again.
Princess Yuna: It's gone.
Snowdrop: Yes, Yuna. It's over!
There is a crash and the ground lurches making them fall off balance. They struggle further up the rock. Yuna collapses onto her back. We can see the lava flow moving all around them.
Yuna is lying back with her eyes closed.
Princess Yuna: We did it together, Chernabog is defeated, we'll always be memorable.
Princess Twila: Even if other dangers awaits us, We'll always be together until the end of all things.
Yuna gets up and comforts her friends and cousins along with Sméagol.
Princess Yuna: Yeah.
Sometime later... the foals are lying prone on the rock. In the distance eagles appear flying towards them. Star Swirl is on the back of Marahute. They fly in and pick Yuna, Her friends and Sméagol up carefully in their claws. We can see Yuna lying back with her eyes closed.