Unfortunately I was raised a Jehovah's Witness (never baptized) and it took a long time after I left to finally rid myself of all their teachings and to visit an actual christian church.
Since I am an Anglican and not a Lutheran, I won't answer how Lutheran's view Jehovah's Witnesses, however, I can give everybody some advice on how to reach Jehovah's Witnesses.
As others have mentioned, JWs are very hard to argue with about theology since they talk in circles and have little interest in what you have to say and why you believe what you believe. Everything they believe is dictated by the governing body located in New York. Everything contained in the bible is to be read with an aid that was written by the governing body and those who work with them in New York. They are not to disagree with any of these teachings.
In order to actually reach Jehovah's Witnesses you must shake their faith in the governing body as the governing body is everything to them (God's mouthpiece on earth). Fortunately there is a plethora of information to share with JWs about the governing body that is sure to at least get them thinking.
First of all, one of their most prized booklets is the "Should You Believe in the Trinity." This booklet is packed full of false quotes and sources that can be easily shown to have been taken out of context. The amount of misquoted sources is truly staggering. Ask the JWs "if the Trinity is such a false doctrine, then why did the governing body misquote and misrepresent so many sources in this booklet?" I have actually printed the information off of the second website and will show it to them, they are usually left speechless. The governing body does this stuff all the time and is rampant in most of their publications.
Watchtower misquotes regarding the Trinity
Examining - "Should You Believe In The Trinity" a JW Booklet
Another way to show the JW the errors of the men in New York is to show them all the failed prophecies:
Failed date predictions of Jehovah's Witnesses
Lastly, show them how the governing body has flip flopped over the years on the important issue of blood transfusions:
Jehovah's Witnesses Flip Flop on Blood transfusions!
It won't be easy to show them just how big of liars and deceivers the boys in New York are, as many of them are heavily brain washed. However, even if you can plant the slightest seed of doubt, it can eventually grown and lead them to asking more questions. I hope this helps!