I have a good (in my view) argument against "new light", but it`s not going to work, partly because it`s based on logical reasoning (and logic has nothing to do with JW-ism), and partly because, as you said it yourself, it`s like "banging your head against a brick wall". Ok:
1) "New light" means that God has lied to his flock in the past, and that is impossible, because God doesn`t lie. New Light means that it has become necessary (for whatever reason, usually a failed prophecy, like the generation-change in -95, or the failed predictions of -75), to change the doctrine. That`s what it actually means, to change the doctrine! And if it has become necessary to change the doctrine, that means that God must have lied in the past, because, after all, not telling the truth is the same as telling a lie. They will respond to this by saying that "people make mistakes" and that "the governing body is not a prophet, and has never claimed to be". Both arguments are weak. If "people make mistakes", then what guarantee do you have that they are not mistaken on other issues, or on the "new light". The second counter-argument is a lie, because there are many WTs, Awakes and books, some aren`t even that old, that proclaim the WTb&ts as "Gods prophet on earth in the last days", or "Gods prophetical organisation". However, this line of thought is to complex for the average JW-mind. Also, and this is where it gets really complex:
The term "New Light" indicates that that is just what it is: A "new light", that is, a term that does not relate to pure reason alone, but to a "new understanding of divine origin". That is, the new "understanding" has been given to the GB by God himself, either thru JC, the holy ghost, or angels (they even claimed that, once upon a time, that angels were in direct communication with the GB). That is really (logically) like saying that Gods words (thru the holy Bible) were not the "final say on the matter" (exemplified by it being necessary for them to come up with their own Bible translation, the NWT), but that God is in direct contact with humans, and that his revelations (ie his BIBLE) is "not finished", that it is a "work in progress". This is really dramatic, and indicates that the GB is not just a group of christians/bible students, studying the bible, but a group of prophets in direct contact with GOD! And this underlines the original argument i started this post with: Why has God chosen to tell lies in the past? Not only telling lies, but telling them directly to his organisation on earth!? Why? The answer will be "eh, oh, eh, there are some things God doesn`t want us to know yet", or whatever. Forget the whole thing, open a bottle of wine instead..