I must admit I had trouble killing her too. I believe she's one of the toughest monsters around in the first chapter, so I took her out quite at lvl 25-30 or so. Just make sure you have a decent weapon and plenty of super healing potions. I boosted my alchemy skill to 4 and made potions whenever I came across mushrooms, so I had about 15 super healing potions when I fought her. Just keep hitting her ( you really should have stun ) and keep an eye on your health bar. I also left a teleporter pyramid near Zandalor so I could get out and healed. The dreadknight I mostly ignored since he didn't really hurt me, I used swirl attack to take him out, so I could keep hitting Josephina at the same time.
