There is a healing potion in the cell just east of Edmond's cell, one in the NW corner of the room with the talking skull, and you can get another for convincing Edmond he is dead. Barrels have a chance to contain potions, and you can quicksave before hitting alt or running your mouse cursor over them, then quick load until you get something good.
I found speed made a difference; the slower you are the more times Fergus gets to hit you before you can hit back. Other people have boosted strength almost exclusively, for the damage bonus.