Do you mean Josephine (I don't think Iona summons a death knight, but it has been a while)? If so, the 'shield' is actually Blind being cast on your character, so you can not directly target her. You can use a shadow potion to wait out the effect, or switch to area effect skills that do not need a target (like Hell Spikes, which do physical damage, so can not be resisted).
In any case, the techniques that work against Josephine should help with any strong opponent:
Chasing Josie...Do you have the survivor's Curse skill? A few repeated casts of that (even at level 1) can take care of her resistances.
The Burning Wall skill can be very effective, with good timing and coordination (for best effect, you need to cast the spell and run in a tight circle around a target, or group, and it can also be used to slow and damage opponents trying to follow you).
