I took the best case scenario for an attacker (excluding FNP), in the case where Magnus has the 2+ re-rollable inv. save and Invisibility. If the attacker re-rolls his To Hit and To Wound rolls and wounds on a 2+, Magnus has 99.99175% chance of saving the wound. It means that approximately 1 in every 1000 wounds will go through.
That being said I don't think he'll have access to Invisibility with those percentages :)
P.S. Didn't one of the new powers they showed give +2 to invuln saves?