A textbook Uber Shield is a Large Shield of Bashing +5, +5 Defender, with possible Guardian and/or Defiant on there as well. (Defiant is +2 AC vs a creature type, i.e. Anti-Bane, and Guardian is +bonus to Saves, i.e. Defender for saving throws).
Explicitly, you can use the Weapon Enhancement on the SHield to power Defender/Guardian while getting the +5 bonus from the Armor Enhancement also for protection, and use Shield Mastery to also keep +5 th/dmg.
That's WHY you do this, and why it's an uber shield. +12 (+14) to AC is Uberrrrrrrr.
==Aelryinth