Disclaimer: I am bit rusty with DFRPG and have breezed through Fate Core once.
As far as I understand the amount of shifts caster uses on this only increases the amount of free invocations. They are usable for caster only, unless he uses 2 shifts to make it zonewide or uses his own action to defend another. Alternatively, create armor 1 per 2 shifts.
It can be used as a passive obstacle (Block ?) with base strenght of 2 shifts. Or to roll active defence with Discipline instead of Athletics. Caster can use those free invocations on both of these with normal +2 per invocation. Others can use the +2 on their own defence rolls .
With free invocations, once something breaks the shield it is gone and all remaining free invocations are lost. Caster must use supplemental action every round he wants to keep the shield on, even when no invocations are used.
Alternatively, caster can create armor 1 per 2 shifts. How each shift is used must be decided while casting. Armor remains after taking stress and stays as long as caster maintains it.
I dont know what happens when these are mixed and shield is pierced. Probably easier to make it either invocations or armor choice.
Shielding others is strong, but free invocations run out pretty fast agains a group of characters. Either caster uses his own actions to generate more, and takes stress normally from casting, or the shield aspect is invoked with fate points.
Strong, but not broken in my opinion.
Armor might get pretty crazy pretty fast. Lets say 10 refresh wizard, 7 power and control after focuses(focii?) and specializations. With 6 shifts that is armor 2 for everyone in one zone, easily controlled. Other than supplemental action every turn, caster can keep this up indefinitely and still throw power and control 6 spells around.
And that is not remotely optimized. Problem with armor is that other than taking out the wizard, there is not much opponents can do.
What happens when ally, for one reason or another changes zones? Does the protection disappear? What if he returns to that zone and caster has been maintaining the shield?