1. Concussive blow does indeed only proc with a mace-like weapon
2. +x% Stun Damage means extra damage that goes against stun resistance
So if you got a mace that does 100dmg and concussive blow gives you +50% stun damage, you got 50 extra damage that is not affected by armor, but by the stun resistance of the monster...of course only when Concussive blow procs...
I'm pretty sure this is the way it works...
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