Another small trick would be to be sure to summon your familiar just before entering Desther's level (if you haven't already) and then rest. That will provide an additional familiar in queue when the original one dies. Then, prior to battle, buff your toon, possess the familar and try to kill at least one or 2 of the ritual creatures or other creatures in the chamber. When the familiar form dies your toon will suffer only minor damage so then rinse and repeat for another "safe" attack (you'll need to rebuff in between to empower your familiar possession the second time).
I suppose a good bet would have been to buy at least 2 of the magic missile wands from Eltoora. It will probably take more than 1 full one to kill Desther but it is a sure way to do damage from range as your xbow or sling will miss a lot. That unlimited charges ice wand earned as part of the prelude training would work as well (hope you didn't toss it aside
'> ), since Desther only has a bit of fire resistance but nothing against cold.
If you do decide to melee, ask Linu to use her xbow while your toon does the tanking. The AI seems to refrain from her casting spells with a range weapon equipped (at least, that's how it seems to work for me). As long as she is plunking and not buffing or healing, it will help to lower Desther's HP total each round.
You may also want to consider taking shield proficiency to use especially in forced-melee confrontations like this. Yes, it does cost a feat, but can be very useful if no multiclass contains that proficiency. Unlike armor, a shield can be swapped on-and-off during combat so as not ot incur a casting penalty when your sorc is doing offensive casting, dispelling or rebuffing.
Also, a pseudo-cheat, would be to delevel/relevel using the console and select more appropiate spells if you feel you made a bad guess. Take note of your "before" XP tally to prevent adding back too much/too little when you relevel. Personally, I suck playing sorc and have used this to correct poor spell selections I may have made (also, same learning curve for casting bard toons). Wizard is my own arcanist of preference for the full repertoire on demand, but pure sorc gets a better weapon selection than pure wizard does and doen't need to go through all the spell prep business.