There might be other ways other than this but this worked for me. I did this at level one on a 'tank' class with iron armor. I did this with a shield and sword, but it might work with a 2 handed weapon.
When Umbra attacks, block her attack, and try to move behind her, and circle around, attacking her with your weapon. It takes a few tries to get it down. I was able to hit her on average, 3-4 times before repeating the process. I did it on an orc and used berserk, and even then it took roughly 10-15 strikes before she was dead.
There are many ways to kill Umbra at level 1. Some other easy approaches include:
Difficulty right down. (makes it easier)
1. Jump atop broken column with a claymore (also works with hammers and battleaxes) 2. Cast flare at Umbra 3. Evil laugh as you slash at Umbra with your claymore as she swings her sword wildly but still cannot hit you due to its shorter range.
1. Jump atop broken column with bow and a stash of arrows... 2. Shoot her until she dies.
1. Jump atop broken column with a pocketful of restore magicka potions. 2. Cast flare at Umbra until she dies.
1. Buy apotheosis 2. Make sure you don't miss too many times.
--- "But I'm more than just a little curious How you're planning to go about making your amends"