For example if I use a naptha to gain +20 fire damage to weapon attacks and then use thunder strike (+150 attack power) do I then deal 20*2.5 = 50 bonus fire damage on that attack?
Same idea for off hands, if I use assassinate (+500 attack power), does my offhand strike inherit the damage bonus?
where B is the base damge of the attack or spell
DR% = damage reduction after Armor/Ward/Focus has been subtracted from Armor/Spell pen or Temptation
ER% = elemental resistance
Where exactly the bonus dmg from enchants would fit into that formula I'm not certain.
But... from testing I've found the dmg from enchants don't seem to scale with a skill's bonus atk power even if it's a critical hit.
I tested a short sword (60pen) vs the corrupt lupines in the Old Forest on [Normal] difficulty using
[Attack]
(1)Dirty Trick +50 atk pwr
(3)Get Over Here +100 atk pwr
And incorporating [No Buffs] or [Brazenberry Ale] or [Naptha] or [Blood Iris]
From my data set,
[Naptha +24 Fire] added ~20ish extra damage
[Blood Iris +24 Blight] added ~40ish extra damage. Expected since Beastkin have a -75% weakness to Blight.
Conclusion tl;dr
1. [Naptha] doesn't seem to scale with a skill's bonus
2. Offhand attacks don't gain bonuses