First post here, the game said to let you folks know and this seemed a good a way as any.
It was kind of cheap how I did it, basically all I did was use an invisibility potion to sneak down to the end of the little gated off corridor in the lower left corner of the map, from there I inched my way back up to where they would notice me, lured one or two of them down there poking at them with arrows as they approached, and then finished them off with a sword. After that because I was down away from the rest of them where they didn't see me I could camp to heal myself up without using up potions unless they got in several lucky hits and was forced to.
Then I would go back to lure one or two more down.
It took a little while to do this as you might imagine and like I said, it was kind of cheap and I wouldn't really count this as having defeated them outright like others seem to have done. Still, that's pretty much the tactic I used through the entire game, get one or two away from the group, take them out, camp to heal, repeat. This was letting me chug through areas I think were meant for a higher level than I was usually at.
...I spent a lot of gold on arrows...