as a mage on normal i pretty much took her out with flamearrow (with maxed spell lvl and 2/3 higher magic.)
I had miners, dwarves, shamans, cannoneers & beholders. These took care of all the adds whilst i whittled the boss down. Occasionally used blade of axes, salvo and a beholder ray on the boss as they refreshed.
Easier fight than *my everyone on the boss* disaster first time.
I found it helpful to have no troops in melee range of the boss.
Minimal losses this way, ranged on boss, melee on adds