The key to the door should be in a look bag from one of the opponents in the group just before it.
Can you stand back and take out the healers and shaman with a bow, rather than move into/through the fire? A creature with a mage or ranger head might help.
If you do Stone and Raze's flying fortresses first, and don't get around to the imp cave until you are level 31, the fire doesn't do any damage.
Welcome to the forum.