[Radek]

What to do with Xydusa:
1. Use the Scrolls of Magicshield (if you aren't able to cast Merlin's Shield). The Merlin's Shield spell lowers Xydusa's chance of stoning you considerably. The scrolls can be bought in the Mages Guild in the "members only" shop. Any number of the scrolls can be bought.
2. Use the Potions of Heroic Might (if you cannot cast the spell yourself). Both the Valeia Mages Guild and the Ishad N'Ha Mages Guild sell one Potion of Heroic Might.
3. Use the Potions of Bloodlust (if you cannot cast the spell yourself). Again, the Mages Guild sells the potions.
4. If you cannot cast Cure Poison then make sure you have one Potion of Cure Poison for each member of your party (preparation for the worst case). Xydusa is poisonous and her poisoning is really serious: you will lose 5 HP per hit by the poison. Such poisoning should be cured ASAP.