Also drink the best resist potions your alchemist can make for those battles. Have powerful healing ready.
If you cannot kill her in close combat, reset your skills to a more ranged attacker (int 65 should be enough to do some nice damage in the end). The first attack spell is actually a nice thing. Boost your creature and get other creatures (demon! Ghost for healing!) you can summon.
A highlevel ghost will refresh your hitpoints in a truely awesome way.