I found a discussion in the DDO forums that hits directly on the problem we were having in Gianthold as well as with the Fire Gnolls in Menectarun. When we attack, any casters are throwing "Fire Shield Defense" which comes in two forms; a cold and a fire.
another post suggested resistances of +20 to +30 for fire/cold should do the trick.
http://forums.ddo.com/showthread.php?t=171053
ddo forums wrote:fire shield comes in 2 form. the cold and the fire. the cold shield reduces incoming fire damage by 50% and inflicts cold damage on anyone that hits. the fire shield is the direct opposite. if you are hitting on a mob with fire or cold shield, just have the appropriate resistance up and you should be fine
another post suggested resistances of +20 to +30 for fire/cold should do the trick.
http://forums.ddo.com/showthread.php?t=171053