Hello and welcome;
Well trinity is an opinion and it's a weak theory to try to prove. Personally I don't buy into it either. If you want something to get witnesses confused go for the 607BC date as a destruction for Jerusalem. If you prove them wrong on 607 you should easily show that 1914 isn't a date for Christ's return and therefore 1919 is not the date for Christ appointing faithful and descrete slave thus making the jw leaders a fraud and a bunch of liars.
You can ask them something like
"I've read somewheres that you taught that Christ was to return invisibly in 1914 some 40 years prior to that date. Is that true?"
Depending on the dubs they will probably respond "yes" and tell you that it is true. They may show you some articles where it says that wts taught that 1914 would mark the end of the gentile time some 40 years prior. Ask if by preaching end of the gentile times they also mean return of Christ? If they don't show you anything to do with gentile times then ignore gentile times and go directly to how they arrived at 1914 which the basis for will be 607BC and argue from there. If they do show you gentile times article(s) then ask them why they taught Christ returned in 1874 until 1930s? You can stay on this very topic for a while since most witnesses don't know the 1874 teaching of Christ's return. For reference you can use Thy Kingdom Come book which was written by Charles Russell and here is a pdf link:
http://www.a2z.org/wtarchive/docs/1890_Thy_Kingdom_Come_1911_edition.pdf
And then ask them to show you how they arrived at 1914 which as stated above will lead you to 607BC as destruction of Jerusalem by Babylonians which can be shown to be wrong through chronology of kings that wts agrees with and you can use this thread for some good discussion:
http://www.jehovahs-witness.net/watchtower/bible/55372/1/586-587-the-K-I-S-S-approach-no-VAT4956-Ptolemy-Josephus-needed
and if you do some research on archeological, astronomical and other historical evidence pointing to the 586/587BC date for destruction of Jerusalem you should get these witnesses either re-thinking their belief in 1914 or they will leave you alone. Be prepared and don't change topic until you argue it out.
They cannot prove 607BC outiside their own reasoning which doesn't have much legs to stand on. If you can get Gentile Times Reconsidered book by Carl Olof Jonsson it's worth every penny for anyone studying with witnesses or anyone having any dealings, past or present with witnesses. Their reasoning is flawed in a huge way since they use historical evidence for 539BC as a destruction of Babylon by Cyrus and the same chronology they ignore which points to 586/7 and not 607BC.
They may say that they don't serve for dates and it doesn't matter if 1914 is wrong then that's where you have to point out that if 1914 is wrong then the faithful slave wasn't chosen in 1919 and their leaders are liars and are not chosen by Jesus.
This may take you a bit to research but since you're already putting time into "studying" with witnesses you don't have that much to lose and you'll learn a lot about the wts corner stone teaching that makes wts the apocalyptic cult that they are. If you cannot find Gentile Times Reconsidered in local stores you can check here, it will not only give you some good knowledge about wts magic dates but about the history of religious movements seeking the end date.
http://www.commentarypress.com/Publication/English.html
Arguing trinity, immortal soul or hell fire will not get you anywheres IMO
good luck
