I think the reason Dragonfly ask me to post this tip is because a lot of people don't know that it's easier to go after the sword at lower levels instead of waiting to get the sword when you do its quest. To do the quest you have to be level 20.
i do it as early as i can find her and all i do is fight normally. mabe it's just the way i fight her or i dunno.
but i just simply stock up on health potions and dance around her with a dagger and keep on hitting her back, works for me :P
Hey if you guys are offing her with a dagger and normal tactics, good for you. Great job. But for those who don't know, here's a few stats on Miss Umbra.
She is a level 50 Bosmer with 394 health points and can heal herself at the rate 2pts per second. She has 100 Speed, 100 Strength, 100 Endurance and all her combat skills are max out at 100. ref: uesp.net/wiki
Also if you wait until you get the quest, as soon as you enter her area she attacks and she is very very fast. If you go before you get the quest, then you can get in several attacks before she reacts.
The first time I played, I went for the sword after I got the quest. If I remember right, I had to reload my saved game 5-6 times and must have used up 1/2 dozen healing potions.
For what it's worth, by doing it the way I have suggested, you not only end up with Umbra Sword, but also IMO two of the best weapons in the game. Sunderblade and Apotheosis.
Anyway, regardless of how you do it, your best shot is do it before you get the quest.
Good luck.