i ended up finding a pretty informative post for anyone who is curious.
Here are the blizzlike resistances/immunities of raid boss mobs:
Lucifron: 93 fire resist, 186 shadow resist
Gehennas: 186 fire resist, 93 shadow resist
Baron Geddon: fire immune
Shazzrah: 186 arcane resist, 93 fire resist
Sulfuron Harbinger: 93 fire resist
Golemagg: 186 fire resist
Majordomo Exectus: 95 fire resist
Ragnaros: fire immune
Onxyia: fire immune
Vaelastrasz: fire immune
Firemaw: fire immune
Ebonroc: fire immune
Flamegore: fire immune
Nefarian: fire immune
Viscidus: nature immune
Sapphiron: frost immune
Kelthuzad: 200 frost resist, 100 shadow resist
Extra fun resistance:
Ossirian: 1000 all resist
Ras Frostwhisper in Scholo: 186 frost resist
Shade of Hakkar in ST: 125 frost resist
The Unforgiven in Strat: 171 all resist
Roccor in BRD: 153 fire resist, 153 nature resist
Golem Lord Argelmach in BRD: 174 nature resist