I was involved with more than 1 false guru and cult. They are basically all the same. A con artist gives weak people a ready-made ideology belief system and a sense of belonging. This particular NKT cult seems to thrive on negativity and creating an enemy to fight against. And because they put so much energy into fighting the regular gelugpas and the DL, can the outcome be other than a bunch of people, who have been taught to exaggerate and carry grudges? No thanks, I don't want to fight the NKT or even defend the DL. HH the DL doesn't need me to defend him, because he is so far beyond being insulted by them, or harmed by them, that it seems ridiculous to me that he would ever be concerned about his reputation or veracity being questioned. His only concern is working for the benefit of others.
I think a more beneficial method is to teach the dharma about right speech, right action, etc. We cannot change other people. We can only change ourselves. See the NKT people for what they are, suffering people trapped into delusional dualistic thinking and very negative activities. I don't care if they worship Shugden; that is their right. I just don't see that Dorje Shugden practice has helped them to increase their loving kindness, certainly not towards the DL. If you still feel that you MUST DO SOMETHING, study some dharma teachings about right speech. Make your own speech and actions be beneficial to others. Who knows? Maybe a few well-spoken words to an NKT person will have a profound and beneficial effect. Don't set up the target for the arrow.