From Diablo Wiki
When a target is Immune to an attack type, it simply means that damage of that type cannot harm it. The Paladin Conviction Aura and the Necro Lower Resits curse, is some of the ways to deal with this. Having more than one damage type is also an option, and it is for this reason that some players favour to have a dual elemental sorc, instead of a pure.
Copyright IncGamers Ltd 2016