Immune to Element (Fire, Ice, Shock, Poison, Arcane) is on the Enemy Affix List in the Codex > Dictionary. a monster would need all 5 Immune Affixes to be Immune to all DMG.
Effective Mythic is affected by the Element Resistance & Immune to Element of monsters.
Ignore Resist ignores all of a monsters resists & immunes. doesn’t have a Cap, you only need one on your Gears to Ignore all of a monsters Resistances.
Weaken is added to the monsters resistance or immunity when figuring DMG.
this Guide explains how those 3 work.
lets say your Weapon does 1000 DMG. if the Weapon is Fire Element, it is doing 1000 Fire Weapon DMG.
if you are fighting a monster Immune to Fire, your Fire Weapon is doing 0 Fire Weapon DMG instead of 1000 Fire Weapon DMG.
with Ignore Resist, you would do 1000 Fire Weapon DMG against a monster with Immune to Fire.
with Weaken +200%, you would do 2000 Fire Weapon DMG against a monster with Immune to Fire.
basically, you don’t need to worry about monsters taking zero DMG from you on higher floors unless you don’t have Ignore Resist or Weaken when they are Immune to any or all Elements you are using.
it is possible to get to a high enough floor on a Heal Map that monsters Heal more DMG per second than you can inflict on them in a second.