My initial thinking was geared toward a spirit evocation which would act like a ward. The same 'rules' for wards would apply. They can be battered down by exceeding their value. When attacked but not penetrated, the ward reflects the attack at the same value back at the attacker. Just like with wards, the attacker would then have the chance to defend against the reflected attack.
Since the general rule seems to be that special features cost 2 shifts, It hought it would be a suitable place to start.
Another idea to achieve a similar result would be to attach the effect to an enchanted item would carry the ward, when activated it sets up an instant ward attached to the personal nature of the item (say a ring always worn by the character). While not as strong as the threshold concept, there's nothing to say a threshold is absolutely required.
The ward could be tied to the location where it was activated so that the player cannot move from the zone without the ward collapsing. The ward would then just do what wards do.
