One-Shot, Sorcerer 20 needs spell help.

[Draz]

Assuming you're not trying to avoid Polymorph cheese ...
Assuming you're staying away from many useful defensive spells that a Cleric can cover (e.g. Remove Curse), in hopes that your party will end up with a Cleric ...
Assuming your DM interprets Shadow Evocation/Conjuration fairly liberally, like most DMs seem to (but not mine) ...

WAND: Endure Elements (or get magic items that simulate it), Silent Image
L0 (9): ... whatever. Definitely Message, it's undervalued.
L1 (5): Ray of Enfeeblement, Protection from Evil, Feather Fall, Magic Missile, Grease
L2 (5): Glitterdust, Resist Energy, Rope Trick, Mirror Image, Web
L3 (4): Greater Magic Weapon, Haste, Slow, Stinking Cloud
L4 (4): Shadow Conjuration, Resilient Sphere, Enervation, Polymorph
L5 (4): Persistent Image, Teleport, Baleful Polymorph, Overland Flight
L6 (3): homebrew, Greater Dispel Magic, Acid Fog
L7 (3): homebrew, Limited Wish, Spell Turning
L8 (3): Mind Blank, Greater Shadow Evocation, Dimensional Lock*
L9 (3): Shapechange, Time Stop, Dominate Monster

*Wouldn't normally recommend, but in a high-level one-shot where you know there will be lots of demons, it's a different story. On the other hand, for the same reasons, I'm not recommending e.g. Invisibility or Greater Invisibility.

Reflex tends to be the weakest save for demons and dragons, so I gave you all three Core low/mid-level Reflex Save Or Suck spells (Web, Resilient Sphere, Grease). Especially because Resilient Sphere has defensive uses and Grease has utility or anti-grapple uses. Take Heighten Spell for sure.

A few cross-class ranks of UMD probably couldn't hurt, with your Charisma. Then you could keep a couple handy scrolls of Cleric spells (or Glibness or Freedom of Movement) around.

Anything you want, but don't have, you can get on-the-fly via Shadow Conjuration, Greater Shadow Conjuration, Limited Wish, or (at worst) Shapechange.

 

[Runestar]

No one brought up forcecage or maze? Though technically, greater shadow evocation can be used to replicate forcecage without the need for the costly material component...

 

[Shin Okada]

While Shadow XXX spells are versatile and useful, they are often useless in higher level campaigns. As adventures often take place on Innner/Outer Planes (or in Demiplanes).

Read P.62 Column of Manual of the Planes. It's mainly talking about optional cosmology in which shadow plane does not exist. But in the last paragraph, it says "Within the D&D cosmology, the Plane of Shadow connects only with the Astral Plane and Material Plane. The above limitations apply when a traveler is on an Inner Plane or an Outer Plane."

OP is mentioning that he will fight against some kind of Demon Lord. Thus, it may likely to happen that the showdown battle will be fought in Abyss and such.

 

[Quartz]

First off, consider reducing yourself to L19 having cast Wish thrice to get yourself 3 extra spells known for extra versatility and leaving yourself 15.5K XP for casting spells.

For spells, Shapechange, Mordenkainen's Sword, Limited Wish, and Wish should be on your list. Maybe Prismatic Spray. Can you somehow get Arcane Fire (q.v. Archmage Prestige Class)? That would simplify your low-level spells.

As others have mentioned, spells that don't allow SR are good. Think also about control spells like Dimensional Anchor or Lock. You don't want him teleporting away. A shedload of Monster Summoning spells wouldn't go amiss either.

Edit: consider spending XP on having True Seeing as a Permanent spell. Similarly other spells - Permanent Foresight, anyone?

 

[pawsplay]

10,000 xp will buy you 33 castings of limited wish. I cannot imagine NOT doing that for a one-shot.

 

[Quartz]

10,000 xp will buy you 33 castings of limited wish. I cannot imagine NOT doing that for a one-shot.

Ooh! Especially if you use Limited Wish as an Anyspell (Wizard 1-6, Cleric 1-5) substitute. Solves the Healing issue quite nicely, too.