Blessings to you in the name of Jesus Christ.
Um, yes, anyone who might say that JWs are Christians don't really have a meaningful idea of what it is to be Christian. David Waltz is a screwed up man, and I pray he'll repent. Certainly not many RCs that comment on BeggarsAll would say that JWs are Christian, but DW is in a class by himself at this point, to be sure.
There are quite a few Oneness ppl around where I'm from, too. I'd suggest some study over at CARM.org, and Matt Slick the CARM guy has done a few radio shows with a Oneness person, interesting listening.
Of course James White's The Forgotten Trinity would be a good resource I should think.
I haven't dealt with too many Oneness people, but I'd ask them if Jesus was talking to Himself in John 17 and in the Garden of Geth. I'd ask them from a different (this one philosophical) angle the same thing I'd ask a Muslim - Is God love? Is He eternally loving? Is love love when it is directed only at oneself? (Assuming they say yes, yes, and no) I continue: So, before God created creation, how could God be a loving God? Was He just loving Himself? He needs creation to fulfill one of His attributes, even one of His most central attributes? How is this possible?
You could ask them about the crucifixion, the death of the God-man. How did that work itself out, exactly, when Christ was on the Cross and yelled out to the Father? When He died, what happened?
Why is it that the NT very clearly teaches the Trinity?
Also, they're generally works-salvationists, legalists. They'll say you have to be baptised in the name of Jesus (and that's the only way) to be saved. Baptism is a pre-req for salvation. That's a problem. Share the law and the Gospel with them.
They'll say that you have to speak in tongues as a pre-req for salvation, b/c it shows that you're filled with the Holy Spirit. that's a problem. Share the law and the Gospel with them.
Those are some initial thoughts.