When you first start out in the game, you want to make sure you grab every single potion when you escaping the imperial prison that you can. Make sure to have heavy armor or and a decent sword the steel short sword you loot from the first blade captain (Captain Renault) that dies is fine.

Now then where were we? Ah! Then what you want to do is goto a place called Vindasel, it is southwest of the Imperial City and east along the road from Clavicus Vile's Shrine. Once you fight the summoner outside and clear your path inside the dungeon you will arrive at the end of your long overdue journey.

Alas there she is lass/lad. Umbra, the female bosmer holding your long deserving reward. Save your game prior to fighting just in case it doesn't work for you (Ive done this a thousand times on multiple consoles including the PC :D) And let the battle begin! Each time she tries to soul trap you wait till you are roughly at 25% or wherever you feel comfortable and use a potion. Keep pressing on and make sure to avoid her heavy swings it is easier to swoop in and back out when attacking her.

The cursed woman lies on the floor, surround by a pool of blood the only untainted possession she carries is the thing you wanted all along! Umbra is unscathed and you shall take it as your eternal reward! Congratulations hero!