Magical Abilities-Defense
Rocks like a rocking thing.
Canon shows Yuuno's shields failing twice. Ever.
Once during traing, when Nanoha's attack's latest upgrade comes with a shield bursting extra and it's a surprise to everyone. Second when he tries to contain all the protagonists from the first two seasons in a mock combat. This includes four several thousnad year old combat magic thing-a-ma-jigs (I'm open to better descriptions for the Knights), three girls of mass destruction, a familiar and another boy genius at combat. While backing up his side in the spar. This is where he shines.
No Milli-Canon had been established yet in this category, except that a Round Shield should be able to stop a Heartless, while drawing it post-haste.