Im still very new to the game, only level 30ish, so this is totally based on theory crafting.
The best is a Nether creature, you need 3 traits to make it “immune” to all form of damage.
Spiral Ward - This creature is immune to damage from spells
Faith’s Armor - After this creature attacks or is attacked, it gains Shell
Vigilance - This creature intercepts attacks on its allies, but when it does, it takes extra damage equal to 10% of its Maximum Health
Artifact need: Heirloom of Sheltering - Start Battle With Shell
So Spiral Ward make you completely immune to spell damage. You start with shell, it blocks one instance of damage regardless of value, with faith’s armor, every time you are attacked, you lose a shell and gain another one. Vigilance just make sure the tank gets the job done.
You can get an attack blocking only version to work on a regular Holy Crusader or Apis Defender. (yes, the enemy can remove buff and one shot you)
PS: I’m currently farming materials to put this to a test.