Question about elementalist

I want to ask witch has mastermid of discord and it said pen 25 fire resis if i used a lighting skill, if I use hearld of thunder on firestoem , will it works too?

It's "If you've used" so I believe that Herald of Thunder will only trigger the effect when you turn the Aura on, not whenever the effect of it happens.

Edit: However, "Lightning Warp" paired with "Reduced Duration" is one of the better movement options for Spell based characters who aren't using Daggers to get Whirling Blades, and it would count as a Lightning Skill for the effect.