Its a difficult fight. Concentrate on one head - you only need to destroy one to kill her. The best method I found was to go and buy a load of health potions, get close for a lock on and circle as fast as you can whilst firing at the same time. Retreat and heal then repeat. Some say you can hide under the rock bridge, the lightning balls can't harm you there.
BTW, if you did the city of the damned sidequest before leaving Surdana, use the sonic breath weapon rather than fire for a slightly easier fight.