Do you have a spare skill point to put into Flash Attack? If you break her concentration before she completes summoning the death knight, she will keep trying to summon, and not Blind you or use Hell Spikes. I just ran up to her when she stopped talking and started whacking (no Frost, but I had Stun), she ran away a few times and kept trying to summon, but I never gave her a chance to complete the spell, and didn't loose a hit point.