Fast travel to the Imperial City West Gate. Exit the gate and cross the bridge to the main land. After the bridge turn left and fallow the path that runs closest to the water. Keep goin. You will pass a few ruins Eventulay on the right hand side there will be an entrance to an under ground cave thin. It will be garded by a spell dude but he is really easy to kill. Go through the entrance to the cave. Go through all of the rooms.
When you get to the last room in the cave you are on a balcony and she is at the bottom. Shoot her with the bow, unequip all of your armor/weapons and start running out of the cave. SHE WILL FOLLOW. Follow the road back to Imperial City. KEEP RUNNING.
She is very fast and you will have to heal yourself while she is hitting you in the back. It is best to have a few potions of health on you in case you can't keep up with healing spells alone. Run into the city and avoid her by the gates until she starts fighting the gaurds. This will happen almost immediately. The first time I did this I sat and watched her take down 5 gaurds before they finally took her down. I did this with my first character who was a pure Redgaurd fighter and was able to make it with a fairly low speed. I am going to attempt this again this week with my new stealth character and expect it to be a little easier due to my high speed attribute. I hope this helps.
P.S. Umbra is a great sword but the stats on it go up as you get better with blade.
P.S. Umbra is a great sword but the stats on it go up as you get better with blade.