In D2:ED I had to re-do this fight a couple times (on level 4 or 5), having saved on the stairs in the cellar. After drinking the dexterity potion I had, the rest of the fight was mostly determined by my initial Rush Attack - if I got the first hit in and kept attacking, I had a pretty good chance of winning; if I missed the first attack (if Penumbra was moving, etc) or a couple later, then she generally won (without the dexterity potion she pretty much always won). After doing the quest both ways, I had to try the fight a few times to see what kind of loot I could get.
For a melee character, level 1 of Evade is helpful (at least in D2:ED additional levels gave less of a boost per experience point, so I did not invest any more).
... and first summon my Ghost, he's my most helpful buddy. 