I think Shadow Evocation is usually used as an offensive spell and the rules cover that quite well.
However, my NPC is about to use it to protect himself from a whirlwind of flying glass that an Air Element is about to create (by placing himself in a Resilient Sphere). He'll weather the storm and dismiss it when it suits him - or so I thought.
Disbelieved Shadow Evocations are only 1/5 effective. But who has to disbelieve the illusion, the Air Elemental or the mage that summoned the Air Elemental?
Or do the glass shards simply penetrate (with 4/5 effect) the sphere because they're objects:
Objects automatically succeed on their Will saves against this spell.
N.B. Shadow magics are "partially real" according to the magic rules:
Shadow: A shadow spell creates something that is partially real from extradimensional energy. Such illusions can have real effects. Damage dealt by a shadow illusion is real.