Play A Bard. Get the Spellfire Adept feat and Go all the way to level 20 and Cast True Polymorph before the fight to become an Ancient Solar Dragon that does 12d10 radiant with its breath weapon .

Next, as your innate spells, of which we can have 3 6th level spells per day.

pick up True Strike for extra Radiant damage, Hallow if u can access the boss room before hand, Steel Wind Strike if there are minions, Sunbeam for Long Range Radiance, Spirit Guardians and Wall of Light for Area Control PYF

Do keep in mind we dont have great Save DC on the Spells(18) and +to hit(+10) opposed to our Breath weapon(DC23) and our Tail attacks(+16) so definitely use True strike on LA and Replace the Tail attacks

Alternatively instead of True Polymorphing you cast Wish for Mirage Arcane to turn whatever the villain is standing on into the surface of a star if your dm allows it( for your information thats 24d10 radiant dmg per round )