Unitarians claim that Zechariah 12:10 (they will look on me, the one they have pierced) cannot be used to prove that Jesus is God. Response?
Actually, there are two messianic prophecies in Zechariah which prove, unambiguously in my opinion, that the Messiah Jesus is God. Those are Zechariah 11:12-13, in which God says that 30 pieces of silver is the price at which God (me in this passage) was priced. In other words, this messianic prophecy is a prophecy about Jesus who is God. Then there is Zechariah 12:10, which also unambiguously says that the Messiah/Jesus is God as well.
However, I believe you are misunderstanding the arguments of the Unitarians. Unitarians do not deny that Jesus is God. What they deny is that Jesus is a second “person” of the trinity. Unitarians do not teach that Jesus was a human being. They teach that Jesus was God. Their heresy is not to deny the deity of Jesus, but to deny the reality of the trinity. That is why they are called Unitarians. Unitarians as opposed to Trinitarians! These Unitarians are not denying that Zechariah 11:12-13 and Zechariah 12:10 is calling Jesus God. What they are saying that the Father, the Son and the Holy Spirit are one God, but not three persons. This is sometimes called modalism. The Father Son and Spirit are three different forms of the one person, God. I am not saying I agree with the Unitarians here, but I am saying that you are probably misunderstanding what they are trying to say.
In other words, Unitarians are opposing the idea that Zechariah 11:12-13 or Zechariah 12:10 support the idea of trinity. In my opinion, these passages are AWESOME proof that Jesus is God. I believe that they are also pretty good evidence for the trinity, but these are not the principal passages I would use for the trinity.