A suggestion I have for being able to kill cold immune monsters without respecing is this:
You can always upgrade your mercenaries gear. Try getting a high defense armor shell and make either a stone or a treachery. I always liked using a treachery because you can buff fade on yourself aswell as your merc which will massively pump your resistances aswell as give you some damage reduce if I'm not mistaken. Stone will add 250-290 % enhanced defense. Hopefully this bonus will allow your merc to take more damage. Try getting a high damage weapon to make an insight and you can sit back, cast some static, and let your merc poke everything to death. I have used this on hardcore mode in nm and hell difficulty which has generally been an effective method to taking out immunes. As long as my cold sorc had a pretty godly merc, countess & summoner were a walk in the park. Nith is a different story sometimes he was a big pain and I didn't really risk the chance of dying to him since I was on hardcore haha. Always remember to pack some potions so once your merc loses about 1/3 life you can give him a super healing potion and he shouldn't die. You can also hire the merc that will add either prayer to heal himself or defiance to give even a little more pump in defense.
I hope this method works, if you are already have gg gear for your merc then look for other ways to upgrade it. If you respec to fire then you will eventually come across the roadblock of dealing with immunes in hell. the goal would be to ideally do every part of hell with little challenge as possible so that is why I suggest focusing on your merc to handle those immunes for you

Edit: I always enjoyed and found some decent stuff in hell cows you may want to try there aswell

Any additional questions you can shoot me a message.
This post was edited by RatM on May 18 2013 03:31pm