Phase 1: He sits on his throne summoning large amounts of monsters/servants to attack you. The intent is not to kill you, but weaken you and exhaust your resources by zerging you.
Phase 2: He then sends 3 of his most powerful champions at you.
A Breton vampire assassin with high magic resist that is weak to fire and moderate physical defense, wielding 2 daedric daggers and using stealth.
An undead Nord champion with 2-Hander that has high physical resist but is weak against magic.
An ancient Altmer mage that uses purely destruction spells, lightning and frost. Weak to physical damage and magic both but frequently uses dragonskin, ward and teleport.
Phase 3: When 2 out of the 3 are dead he joins the battle himself.
Abilities: Teleport, dragonskin, Grand healing, summon dragonborn DLC new creatures of choice with twin souls perk, destruction spells (lightning focused).
Shouts: Marked for death + unique unrevealed dragonborn shouts.
Phase 4: At "death" he casts you aside with fus ro dah and casts off his robe + helmet revealing his face and revealing armor underneath, he becomes a melee fighter.
Phase 5: He mounts a dragon and you fight the last battle on dragons in the sky.
Phase 6: When his dragon dies it crashes into the ground, he transforms into an avatar form of Hermaeus Mora (If indeed he serves him?), a vile corrupted dragon/humanoid hybrid.
Thats how I would go about it. How would you design the final encounter? (From a pure gameplay/combat perspective)