I'm a lvl 24 frost mage trying to solo Ol' Sooty a level 20 elite and it seems to have really high resistance for spells for some reason. After 10 attempts I have averaged the following:
Frostbolt: cast 97 resist: 36
Fireblast: cast 13 Resist: 7
Frost Nova: cast 28 resist: 18
This seems like an incredibly high number of resists for a level 20 elite mob but it may be working as intended