Here is how Star Swirl returns with reinforcements and The Flooding of Jafar's Tower goes in Yuna and the Three Kingdoms.

It was a crack of dawn. Princess Luna looks to the window. He hears Star Swirl's voice.

Star Swirl the Bearded: (voice over) Look to my coming at first light on the fifth day. At dawn......Look to the east.

Princess Luna: (hearing Star Swirl's voice) We must stand until Star Swirl comes with help.

Hiccup: Then let's make this last fight!

The door is broken down

Hiccup: Let this be the hour when we draw swords together.

Shrek leaves to blow the horn.

Hiccup: Are you ready, Bud?

Toothless: (nods)

Hiccup: Fell deeds, awake. Now for for ruin and a red dawn.

Astrid: Wait for the horn to be blown.

Human Rarity: Right, Astrid.

John draws his sword. Hiccup puts on his helmet. Shrek blows the horn. 

The Orcs, Goblins and Uruk-Hai break through the inner door. The Vikings raises his sword in the air

Hiccup: For Berk!

They vikings ride their dragons through the masses through the hall, killing those in their path. Shrek sounds the horn again, as they leave Berk and ride onto the causeway where thousands of Orcs, Golbins and Uruk-Hai still crowd.  Suddenly Princess Celestia and Princess Luna is drawn to a light and looks up. Star Swirl appears on top of the hill top next to berk riding Marahute who screeches

Princess Celestia: Star Swirl.

Star Swirl the Bearded: (looking down on Berk. Hiccup pauses and looks up) Hiccup stands alone.

King Solar Flare: (flew up behind him) Not alone. (he raises his sword high) For Equestria!

The Equinelantians came up behind Solar Flare. The Dark Armies stop fighting and look up.

King Solar Flare: To Berk!

Solar Flare, Star Swirl and the Equinelantians charged down the steep hill towards the dark armies who surge forward holding their spears in front of them. As Star Swirl nears the armies he sends out a bright white light from his staff, blinding them.  They ride over them and into their masses, killing and fighting.

Meanwhile, Yuna and her friends battle in Jafar's Tower with the Maximals and the Skylanders.

Princess Yuna: Take this! (use magic to throw stones)

Kaos: A hit! A fine hit!

The foals throw the wooden towers into the pits. Some orcs use flamed arrows and Wildfire use his traptanium shield. Jafar from side to side very anxiously. The Maximals blast their guns at Isengard. Rhinox pulls at the supports holding the damed river.

Tigatron: Break the dam! Release the river!

Rhinox pulls again at the supports and the water starts to flow through the wall as the dam collapses. Millions of gallons of water rush down the hillside towards the Tower as the Skylanders and Maximals move out of the way. Jafar looks on.

Pound Cake: Pumpkin Cake! Hold on!

The foals brace themselves against the rush of water

Spyro: Hold on!

The water fills up the pits and surrounds the tower with water as Jafar looks on.