I had played Archer C3 -> Wugushi C2 in iCBT2.
Like he said
the Arde dagger or any other sub-weapon with element attack will add additional damage to your skill and AA.
Therefore, the skill with multi-hit will benefit from it. It always works like this since CBT.
For Cafrisun set, It is the only item set that make you additional hit. Therefore, Arde dagger affects it.
Also, the set didn’t give you another hit to your skill. This is also the same for Cleric’s class skill like Sacrament, Blessing or Wizard’s Enchant fire.
Those skills will add additional hit to your AA only. However, when using skills, the damage from buff that said Element Property attack will add to your Skill instead of making additional hit.
For example, Sacrament give you
Holy Property Attack +40
Additional Damage +40
The Holy Property will add to your skill damage (+40).
Additional Damage will only add another hit to your AA, which mean 40+40 = 80 damage (and another 40 damage to your normal hit if I remembered correctly).
It had been disscuss here
I believe those buffs can also stack to each others.
For example,
Additional hit from Sacrament to enemy that is neutral to element will equal to
80 (Sacrament) + 100 (Blessing) + 34 (Enchant Fire) + 154 (Arde Dagger) = 368
Additional hit from Blessing
170 (Blessing) + 40 (Sacrament) + 34 (Enchant Fire) + 154 (Arde Dagger) = 398
Additional hit from Enchant Fire
68 (Enchant Fire) + 40 (Sacrament) + 100 (Blessing) + 154 (Arde Dagger) = 362
If you have Cafrisun set then it will add another hit.
You can correct me if I am wrong.