I suppose the best thing would be is to acknowledge that indeed they do have a point, and do not try to convince them otherwise. An appreceation of the deity of Christ is not achieved soley through a tiresome process of bandying about an x number of proof texts, especially if one uses OT references. If indeed it were so easy, then we would not have Unitarians. There has to be something... I dont know.... a "feel" an "awareness" It simply is'nt a mind game. Its deeper, and it took me a hell of a long time to accept.
Probably the best approach is to accept the biblical limitations. So Christ is not called ALmighty God, especially in the OT. The point is to accept what the text at Isa 9:6 means when it refers to Him as "Mighty" God. As a Jew, who did Isa believe the "mighty" God to be? Not what the followers of the current institution called the WTS, here in the 21st C make it mean, but Isaiah himself, 2600 years ago. Who was this mighty God to him?
For instance he prophesied that a remnant of Israelites would "return to the Mighty God" [10:21], who is also the Yahweh of verse 20, called the "Holy One" of Israel. We, from the standpoint of the 21stC could argue, with at least some justification, that since Christ is the "MG" of Isa 9:6, then this must be Christ here.
I find the adjective at vs 20 interesting. It was once pointed out to me that Yahweh here is only called the "Holy" one. He is not called the "Holiest" One. Ask the WT follower if there is a difference. In fact the term "holiest" does occur, once, in the NWT at Ps 46:4, and it does NOT refer to Yahweh! So, since He is not the "Holiest" One, simply the Holy One, then He must also be less than almighty since He is also simply called "Mighty" God at 10:21.
The interesting thing at Isa 9:6, is not that Christ is called "Mighty" God, but that He is called Mighty "God" Did those Israelites of old bandy the word "God" around so carelessly that it could mean anything that the WTS wants it to mean? Is there a difference between being a "Mighty" God, and being an "Almighty God"? If there is, then why leave it at that adjective? Hmmm? what about "Holy" and "Holiest"
I mean could Yahweh simply be a "nice" god and not be the "nicest"? Let the WT follower know that making artificial distictions in adjectival expressions can be a two edged sword.
Oddly enough, there is a text that has foxed the WTS and we might as well use it. As with so much about the Deity of Christ, it concerns the Gospel of John. [5:19] With almost unrestrained glee they will accept the first portion of this text because the Son says He does nothing of "His own initiative", suggesting, to the WTS system of thinking, some sort of inferiority on the part of the Son.
But let us continue on... because in the latter part of the verse, never actually commented on by the WT, the Son says. "For whatever things the Father does, the Son does, and in like Manner" Hmmm... hold on. Am I hearing right? You mean The Son can do what the Father does? But the Father is Almighty, is that not so? How can the Son possibly do whatEVER the Father does? Not some things the Father does, not most things, but whatever things. The things that the Father does is predicated on his Infinite power. But if, as He says, the Son can duplicate that then He must be .....No?
To suggest that the Son does only what the Father permits him to do is Interpreting the text, not reporting it. The point is simply that Jesus did not say I can do Whatever the Father permits me to do, and no more. So where the first part of the text rightly points out the subordination of the Son, it also rightly points out His equality of ability
Anyway, as I said it is not just a question of proof texts. The WT will never cease from inventing texts to deny the Deity of Christ. Don't be surprised if they say... "Ah but Christ is simply "A" Mighty God, but Yahweh is "the" Mighty God" etc it will never end.
Cheers