Use the Conjure Water spell. The fire won't spread until you break the doors open, so save the Duke's counselor first, and then go to Benryn and grab him, conjure water your way out. My bigger issue is with the atrocious pathing and AI in this game causing NPC's to get stuck or not follow you as they should rather often.
Or your companions or NPC's walking right through your druid's Moonbeam and getting angry, etc.
I've never tried saving the counselor first. I'd have Lae'zel pick up 2 or 3 water barrels to put out the fires, but I'd get only 2 seconds of reprieve before the ground that is now wet is burning again.
Yes, the terrible pathing and AI compound the problem of the mismatch between the environment and UI and dice-rolling mechanics. There shouldn't be any dice-rolling in a time-sensitive portion of the quest at all.