Are you saying that you are getting a bodak in the same ritual chamber with Desther? Seems odd to me. In all the times I've played through the OC I've never once encountered a bodak there. Level 9 is about par for a solo toon who has not multiclassed (re: ECL). I usually either enter the chamber level as level 10 or reach it thereabouts. Bodaks have a relatively high CR so if you are getting them in unison with Desther, it could be just a turn of random luck.
However, I remember a few popping up in the courtyard and then again in one of those side rooms near Desther's chamber and in those cases, follow MM's suggestion about Death ward. It lasts a long time so should remain intact for all encounters on the no-rest level.
Those are shadow fiends sapping your strength, easily remedied with either scrolls or potions of lesser restoration.
...and that's how I solo; use mainly potion buffs to clear the adjacent rooms (usually with a divine favor thrown in to assure non-resistant damage) and then saving most of my slotted buff repertoire entering Desther's chamber. I usually stock up a stack of most of the ability buffs, Aid & Bless so always can have those effects active.
Here's a strategy that should work or at least give you a better chance:
Since you already know ahead of entering the level you are without a hench, I'd slot either a Dire Spider or Dire Bear summon and have it Stand Ground in the hallway near the left door. Potion-buff summon with any ability buffs you have (like Bull's, Cat's, Endurance, Owl's), barkskin, speed... whatever will help it survive the longest. Then a single Invis potion on self so you can run to the back of the chamber undetected and take out the rear ritual creatures and start moving forward toward the door again staying out of Desther's reach. Once all the ritual critters are done, Desther will pursue as will any of the undead left. Pinch them at the door until all are finished off. If Desther doesn't automatically chase, hit him a swat or two and then move into the hall around the corner. When Desther appears around the corner, command the summon to attack. Use a potion of bless right around that time to help the party's attack.
Naturally, since you saved most of your slotted spells for this battle you should be in decent shape. Should Desther use up his single dispel with success, use another Invis, rebuff and re-engage. You won't get any combat XP for Desther so a summon (or hench if you had one) will not drain it.
Knock off about 200 HP from Desther and he will yield.
IMO the key to making this combat very easy is selecting the right buffs to slot, ignoring all those that potions or scrolls will provide. The other is how well you stocked up inventory before entering the final level. I hope you have a fairly high base WIS because the more slots and easier to beat the Sanctuary check if he casts it, the easier to manage these no-rest areas. And the Travel Domain will begin to pay dividends if you took it.
Good luck!
