If you attack immediately after the conversation ends, and interrupt her summoning the Death Knight (the passive Stun skill helps with this), she will keep trying to summon, and not move on to attacking with Blind and Hell Spikes.
The easiest way (ie no challenge left at all) for a warrior to defeat Josephine would be to use a shadow potion, stand near her, then repeatedly use the swirl attack by clicking on the ground (only direct attacks will turn you visible).
Chasing Josie... techniques for defeating Josephine
Hell Spikes does physical damage, so you would need to use Aura of Guarding for a little extra protection, rather than Spell Shield.
