Okay, so it's been tested and confirmed that weapons with a UV damage bonus against slimes does increased damage to the Royal Jelly (Jelly King, whatever you want to call him).
So: We think Vanaduke's a Fiend, what with the slight Piercing weakness. Do UV damage bonuses vs. Fiends hurt him more?
The Tier 1 Boss they're working on is going to be a Beast. Maybe UVs versus Beasts will become a lot more popular once that's released.
Vanaduke is not a fiend. He has no weaknesses. Piercing weapons will appear to do bonus damage [Yellow/Orange numbers and stars on damage] due to a glitch. Same things happens with Gremlin Menders when attacking them with piercing damage [it appears as grey, implying they resist it. However it does as much damage to other gremlins]