One thing I always do to get that fight easier, is to first avoid captain Heagan.
When you come up to the slavers, go to the left, and not against the captain.
That makes the fight easier for me.
But if you still cant do it, I suggest that you do other quests first, and then come back and slay them in the higher levels!!! I can promise you, that is very much fun!
-Mortaug,