Ashadalon in 5e. How would you run this encounter?

[Falcon X]

I'm planning to run Bastion of Broken Souls as a capstone to Tyranny of Dragons. I've already worked out how the party will get involved and am planting seeds early on. The biggest question is: how would you run the final fight?

Note: I have pre-teens playing this, so I'm steering away from tactical combat and towards a puzzle or story-based win.

Setup: Without getting too spoilery, you end up in a cave in the positive energy plane. One of the party members has the correct bloodline to kill Ashardalon. He has minions around, including a few fiends and positive energy beasts.
He is also beyond Ancient Fiendish Red Dragon status.
Obviously he has a breadth of spells at his disposal, though 5e limits him to a single concentration, instead of the original 3.5 version having a whole set of powerups at his disposal.

I've considered making it a little puzzle-like. Perhaps if the Descendent player hit him with a special attack (Magic sword? Inborn power?) it would temporarily disable him while they do things like fight his minions, drop stalagmites on him to breach his natural armor, or maybe turn the positive energy of the plane against him.

What do you guys think? How can I make this fun and engaging for the kids?

[LumenPlacidum]

Ashardalon's fiendish nature makes him powerful, but is ultimately killing him. In the positive energy plane, the effect is to just barely keep his hitpoints in balance. If the party can heal him for a hundred hitpoints, or if the bloodline PC can get some of his blood into the dragon (even if the dragon eats him), it will tip the scales in favor of the healing effects and the dragon will explode in radiance due to the energies of the plane.

[Falcon X]

Ashardalon's fiendish nature makes him powerful, but is ultimately killing him. In the positive energy plane, the effect is to just barely keep his hitpoints in balance. If the party can heal him for a hundred hitpoints, or if the bloodline PC can get some of his blood into the dragon (even if the dragon eats him), it will tip the scales in favor of the healing effects and the dragon will explode in radiance due to the energies of the plane.
Beautiful analysis. I’ll put that at the top of my ideas list.

[Rukelnikov]

I played that adventure almost 20 years ago, and, IIRC, Ashardalon was consuming the unborn souls in order to keep himself alive, that meant he had to stay put in a specific place (the column of souls). Maybe make it so that if he isn't there he starts taking damage each turn.

Another possibility would be... To banish his heart. Ashardalon has a Balor for a heart, maybe if the pcs know this they can try to banish it leaving the dragon heartless and dying.