What class are you playing, and what skills are you using?
Do you have any spare skill points you can use to try out other techniques?
Are you using your creature? I found it much more effective with a ranger head than warrior, and best with a mage head.
You can always leave the fortress, level somewhere else (Raze's fortress, or Broken Valley, etc) and then return for Stone.
When I first encountered Stone I was level 28, the same as him. I figured I would clear up some of his henchmen first, but concentrating on them did not work out very well. After reloading the save I made just after entering the headquarters, I Rush Attacked Stone and focused on him, until he ran away, at which point the remainder were easy to clear out. In Stone's arena (you have to fight 4 other opponents before being able to confront Stone again), the third creature brought me up to level 29, and Stone wasn't terribly difficult after that in a one on one fight.
