Withermores Soul Jar is pretty tough, also you must have entered the fort already to even get there.
Not really that hard - you don't need to fight anyone. You can sneak into the fort in a couple of ways without ant fights and as long as you only interact with his soul jar and not the others you don't have to fight there either.
The silent monks just above are XP600 each (IIRC) which may help although even individually they are pretty tough if you are under leveled.
Level 4 seems to be a big turning point in the game as it is when the decent skillbooks are available to buy or steal.
