a certain roll on a map

SO iv just been running a Unqiue map (Maelstrom of chaos) and i got a roll on the map, "Monsters are immune to a random Element", so i cleared the map pretty well, i went and fought the boss, i killed "Mervil's" 1st phase, then on the 2nd phase the Boss became immune to my Damage..... this is such a corny mechanic please look into changing this even if its changed to a % of Elemental damage, because as a fully fire elemental build what other options do i have to kill this map? i now need to ask global if someone cant come and hold my hand and help me with this map because the game decided to make a boss completely immune to my damage i really hope this isnt going to be a growing trend where half way through a map ur damage is now nul void