Curious, why Mild Wind then use immat arrows? What happens to the element?
When using Mild Wind, it applies a property to both Status ATK and Regular ATK.
Status ATK and Regular ATK are both Neutral by default. Arrow or Weapon Properties overide the Regular ATK. Converters or Endows override it further.
But the only way to change Status ATK is via Mild Wind.
So you use mild wind, which makes your Status ATK elemental which means versus ghost it does more damage. You unequip your weapon to remove the Regular ATK property and make it neutral. Then you use immaterial arrow to make it ghost property. So this way you are doing more overall damage.
Fenrir Example:
Ghost Level 2
Immaterial Arrow=
Status ATK 50% effective
Regular ATK 150% effective
Mild Wind=
Status ATK 75% effective
Regular ATK 75% effective
Mild Wind + Immaterial Arrow (with unequipping the weapon to remove the mild wind from Regular ATK)=
Status ATK 75% effective
Regular ATK 150% effective
Also, Mild Wind changes the property of your Warg, so his overall damage would go from 50% to 75% which is another big chunk.
It can be around a 20~30% overall increase.