I do not know what weapon set you have in your second slot, but one thing you may try is to find and buy a wand with Lower Resist charges. LR's effects will add on top of those of Conviction so you MAY be able to break cold immunities like that. If you do, then your cold mastery will take over.
It takes some time, but you can buy such a wand -- or find it. When you encounter cold immune mobs, try and cast LR on them; the most I've seen (and use) is level 2 LR, which is 37% on top of the 85% provided by Infinity. That amouns to 122% total, a fith of it being 24%, which is enough to break 120% cold resistance. At level 1 LR, you are looking at 31%, 85 + 31 is 116%, a fifth of it, rounded down, being 23% -- again, enough to break 120% cold immunes.
Otherwise, well, you can go my way and trielementalist... No stone left unturned, everything falls eventually
