For one, you need flasks of electricity resistance tonic (buy the recipe in the VR book shop if you don't have it. Two use mark of the rift whenever possible to slow the dragon and, drain her health decently. Three don't let your party randomly attack the dragon, focus all on one leg, until it's out of use, then the other either front or rear leg, and so on until the dragon is dead. And fourth too may mages -
The Gamordan Storrmrider is resistant to electricity and weak to spirit damage so, you only need one mage that can deal spirit damage and, prefers mind blast. Then a ranged rouge, that one will be able to stay alive best and can revive others. Resurgence is good to, once the pots are gone, that can save your team. Also set behavior to have a 0% mana/stamina reserve for all, that lets them use abilities as much as possible.