Ok, so I've been wondering about this since I've happened to get my first 4*...
What if:
The Silversix/Blackhawk is just partly piercing and partly elemental/shadow.
---Silversix Situation: I used it on a Construct. Part Piercing makes it weak against the Construct and Part Elemental would make it strong against the Construct. Would Elemental damage take over the whole damage; or negate the Piercing and make Normal Damage?
---Blackhawk Situation: I used it on a Jelly. Part Piercing makes it weak against the Jelly and Part Shadow would make it strong against the Jelly. Same question in the Silversix Situation, replace Elemental with Shadow.
On average:
Weaknesses give a 30% damage boost to a weapon of the specific damage type
Resistances give an 80% damage penalty to a weapon of the specific damage type
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Old Silversix vs Constructs: piercing part suffers 80% damage reduction, elemental part gains 30% damage bonus
Old Blackhawk vs Slimes: piercing part suffers 80% damage reduction, shadow part gains 30% damage bonus