Card activation...

[Sleus]

Do skills like Rapid Smiting Activates Cards like Hunter Fly card? So for example, I use Rapid smiting on a monster; Is it possible to gain 15% of the damage inflicted on an enemy as HP with each attack?

[XicoR4]

Yes, any skill that does physical damage (ranged or melee) seems to have a chance to activate those cards (there might be some exceptions though, I didn't test every single skill but even Grand Cross and Soulbreaker work on eAthena).
If you mean Rapid Smiting counting as 5 hits and thus triggering the chance 5 times, it counts as a single one (Sub-hits don't count for status or auto-cast purposes).

Also of note is that those kinds of cards seem to stack addictively on both the cast chance and the amount of hp given (tested 20 Sniper Cecil cards and they gave 100% chance to get 400% damage as hp per hit over exactly 1000 tries).

[Triper]

Yes, any skill that does physical damage (ranged or melee) seems to have a chance to activate those cards (there might be some exceptions though, I didn't test every single skill but even Grand Cross and Soulbreaker work on eAthena).

Basically is anything which dmg comes from atk/atk based stat as STR, DEX or LUK. The same applies with cards as Pest/Garm/magnolia/metaler/etc that give status.
Falcon assault, for example, doesn't work with this.
Errende Ebecee Card is a proof of this too since it says at 100% how it works and people most of the time think that is wrong ;(

This is based on eathena ofc, officials can be a little different.