Death Spell:

Sick of fighting hordes of Minotaurs and Umber Hulks? Couldn't be bothered to waste an Abi-Dalzim's Horrid Wilting on another patch of Gibberlings or Orcs? Cast this spell and all of them will drop dead immediately.

Chain Lightning:

Several of these and most enemies will be lying on the ground as charred bodies. Decent damage, and affects an area without risking your party members.

True Sight:

I can see you! I can cast spells on you!

Protection from Magical Weapons:

Assuming you are playing a good Sorcerer, you will eventually gain immunity to Normal and +1 enchanted weapons. Combine this fact with this spell and hey! You are totally invulnerable to all weapons for 4 rounds. A Sorcerer can do a lot of nasty things in 4 rounds. Also, because you are soloing, you will have no reliable tank to distract the enemy while you cast your spells, so this spell helps ensure that you will not be interrupted.

Contingency:

Helpless? Contingency can save you with a defensive spell, then when you are level 18, you can use Mislead instead. Contingency allows you to store one spell for emergency purposes. At level 12, you can store up till a level 4 spell, at level 15, up till a level 5 spell, and at level 18, up till a level 6 spell. You can also use it as a battle opener, storing spells like Protection from Magical Weapons set to trigger when enemy is sighted or when your Sorcerer is hit.

Invisible Stalker:

Skeleton Warriors are far better summons.

Mislead:

In a twist, Mislead has given way to Chain Lightning. I have come to find that the Staff of Magi's invisibility easily replaces Mislead's advantages.

Protection from Magical Energy:

Another good spell, but it is highly likely that you will decimate enemy Mages before they even get a chance to cast most of their high level spells.

Pierce Magic:

While potent in the beginning, it loses its' bite in high level fights. Sorcerers do not have the luxury of changing their spells, so it is better to wait for Ruby Ray of Reversal, Pierce Shield and Spellstrike.

Tensor's Transformation:

A Sorcerer's true power lies in his spells, not in his melee skills.