Character has 350 ATK, the target has 50 PROT. The enemy is strong to fire

The character rune is Double-strike/Fury and Fire Lizard on weapon slot

[(350x1,5)-50]*1.5*1.5=1068.75. How does it interact? I know against unit that is invulnerable to fire, it will deal 0 damage

not sure if Fire Lizard modifies ATK or Damage, but the elemental affinity is applied like other DAMAGE modifiers. normally it's not mentioned because nobody cares about it too much, but physical attacks certainly have "elemental" properties in every game--some games apply separate "physical element" (Slash, Thrust, etc) and "magic element" (Fire, Lightning, etc) multipliers.