there are 2 spells that break immunities: lower res (necro) and conviction (pally). and they only work at a 20% efficiency when breaking immunities. all monsters are given a number value for resistance. if it is over 100, they are immune.
examples: a lvl 17 lower res is -60% (plus or minus two). if a monster is immune, it will be lower res by 60*.2=12. if the monster has 111 or less res, the immunity is broken. if the monster has 112 or more res, the immunity stands.
-res items and cold mastery will work wonders, but only when the immunity is already broken.