Ignore - Nonissue

[Sleuth]

Weapon Damage would be reduced to 0 if the enemy uses Metal Skin. However, if it's imbued with Mystic Reverb, the reverb will still deal 50% of the first hit's original, unmitigated damage.
For example, without any other items affecting damage, if Venator shoots a 120 dmg Ira Domini bolt and Metal Skin reduces it to 0, the Mystic Reverb proc still deals 60 dmg to the affected enemy and ones nearby.

 

[shipress]

I think reverb does spirit damage, and it makes sense it would use pre-mitigation damage since return fire also uses pre-mitigation damage. You would want pre-mitigation damage anyway, otherwise it would effectively get reduced by the targets mitigation, and then the surrounding people's mitigation as well.

Even if reverb is not spirit damage, there is a good chance that metal skin only makes you immune to "Weapon Damage" as in bullets, melee, and the few weapon scaling odd abilities that trigger recharging rush.