Gilthok, I'm experiencing similar results as you, one reflect and approximately 10 unchallenged spells piercing spell turning.
It's abnormal compared to the magnitude of mage fights I've had prior to this, Tenebris included. All of which had superior intelligence and spell level over me.
Spell turning is not perfect however it is quite reliable however in this case, I would rather use steel shards due to the failure rate I'm experiencing.
I'm collecting logs and data to post on prayer so they can try provide an answer as it's not obvious what is different in this case except a certain item and qrace.
Best we keep it on prayer forum as it's not overly constructive posting here.
Ragnomar