been outta the game for a while.
Is cold mastery obsolete with Sunders?
Cold mastery isn't obsolete but it has been nerfed. Basically it can't be used anymore to break immunities. If you use only CM, Cold immunes will still resist your attacks. Sunder charms will drop any immune creature's immunity to 95 res.
Cold mastery & conviction aura will only apply 1/5 of its value to Cold immunes. So if you have -100 to enemy CR from Cold mastery it will turn into -20 to enemy CR. So after your sunder charm takes effect it will drop that creature's CR from 95 to 75.
There is a catch with items giving -X to enemy CR. Those items are not effected by the 1/5 nerf. So if I'm running a perfect doom(-60 to enemy CR) and I have nothing in Cold mastery, then with a sunders it will drop a cold immune creature's CR from 95 to 35. And if I stack my Cold mastery(-100 to enemy CR) with this it will go from 95 to 35 to 15.
So to answer your question its not obsolete. But if you are just using CM without sunders or conviction aura then yeah your CM is obsolete.