How to add damage when a player wears armor

[HypixelsFox]

So i have this fireball wand skript and it has cooldowns and stuff but i feel like its strong against players without armor and too weak against players with armor so i wanted to ask how to add more damage to the fireball if it hits a player with full diamond armor. I was thinking like 0.5 hearts per diamond armor piece.

variables:
{%player%::fireballavalible} = 1

# permissions: skript.Wand

command /wand:
trigger:
player has permission "skript.Wand"
give blaze rod named "&c&lWand" with lore "&kL<reset>" to player

On leftclick:
if held item is a blaze rod:
if lore of item contains "&kL":
if {%player%::fireballavalible} = 1:
shoot fireball
set {%player%::fireballavalible} to 0
wait 3 seconds
set {%player%::fireballavalible} to 1

i'm new to skript so im not really sure how it works and i've tried googling it.

 

[PatoFrango]

Something like

on damage:
    damage cause is fireball
    set damage to 0.5

Not sure if fireball is a damage cause. Try looking in the docs.