If I go pure fire Sorc and use Flickering Flame and Sunder charm will I be able to kill immunes or is Infinity 100% required to kill immunes?

You will be able to kill them, it just may take awhile without something like conviction lowering their res even more

Keep in mind if you post these types of questions in the d2 discussion subforum you may be able to get help there as well and a more thorough explanation, as there are a lot of folks that frequent some of those sub forums that understand a lot of the mechanics and interactions at a much deeper level