Throw shield at enemy. Heavier and higher refined shields do more damage. Damage is 100%+30*SkillLV. Requires an equipped shield and ignores size modifications.
* The damage dealt from Shield Boomerang is always neutral.
Anything that adds or effects weapon-type ATK does not add damage; base weapon ATK, weapon upgrades, Mastery Skills, Spirit Spheres, Provoke, the *Magnum Break bonus, Impositio Manus, etc.
What does add damage:
Attack from STR, DEX, and LUK.
Shield upgrades.
Shield weight (Cross Shield, Strong Shield and Bradium Shield is the best choice).
Damage modifier cards, which all of the above are multiplied by.
Crusader Spirit (via Soul Linker).