Does the Stone Spirit have any vulnerabilities? I have both Hawkes, Isabela and Varric in my party. I don't have any tanks with me. Does the Stone Spirit have any weaknesses (ice, fire, etc.)?
Could it be the "Ancient Rock Wraith" you are fighting?
I only bothered playing through on Casual so I just beat him to death with relative ease, but if you search for "Ancient Rock Wraith" in the following guide it shows weaknesses and some tips...