How many points would you put into cold mastery? Is it better to max it out so you do maximum damage to sundered cold immunes? Or would it be better to max Blizzard/FOrb and their synergies?

Im a noob, please halp

You want -175%. Anything more than that only affects cold immunes which aren't that common, so it's not worth the extra points.

Early on with untwinked gear, you might get more bang for your buck if you only go for -150 res and dump everything else into blizz/orb/ice bolt/ice blast. That brings a sundered formerly-cold-immune monster to -45 res, and everything else to -100. And as you acquire gear with +skills that -150 will naturally increase for sundered cold immunes. Anything more than -150 res ONLY matters for sundered formerly-cold-immunes, and is wasted on all other monsters, so just keep that in mind.

thanks for posting, after 20+ years theres still so much to learn about this game!