Josephine shouldn't be anywhere close to invulnerable; she can blind the character and dish out decent damage (nothing armour, HP bonuses and restoration potions shouldn't be able to handle), but isn't particularly resistant to damage.
What level / class is your character?
Hell Spikes do physical damage, so you'd need Aura of Guarding to add a little protection for that, not Spell Shield.
If you didn't try it, circling opponents with Burning Wall can do a lot of damage quickly, with good timing and coordination, or slow / damage following opponents.
