I've studied JW theology for 5 years now. I've spent a few years lurking and then participating in the ExJw sub (It was full of atheists BTW which wasn't pleasant at all).
People here seem to be giving good efforts, but Jehovah's witnesses are erroneously taught that the trinity is a pagan doctrine inserted into Christianity around the 4th century at best.
That being said these people will do theological back flips and intricate dances to avoid admitting the ever so obvious fact that Jesus is God. You can quote all the scripture you like but they usually serve around it and outright ignore your case, trust me. I've tried argument after argument.
However, I'm working on a new strategy. Forgive my theology if it's poor as this is a new uncharted concept for me. I start by asking if Colossians 1:15 means First in position or First in order/time of creation. The JW will automatically say First in order/time and most likely will refer to Rev 3:14 (I don't bother telling them Arche in Rev 3:14 also means Origin/Source as that will distract them).
Then I draw a line, putting eternity at the start and the beginning of time and space after the eternity line and then another line signaling all of creation. I then ask, "When was Jesus first created?" They'll say "Before time" as they believe Jesus was used by God to bring about all creation.
So I place Him before time existed (In Eternity, a state to which no time exists). Then I ask "How can Jesus be first in order/time of creation, in the state of Eternity (Timelessness), where time doesn't exist yet?" This is where they'll go silent. Here's the thing, in the state of eternity, no time exists. Therefore you can't say "First, Second, Third" in regards to time because First indicates time before second. You can't say Jesus is the beginning of all creation because He would be in a state of Eternity/ Timelessness (No beginning, no end, no time).
If the JWs try to back pedal and say "After Time, then John 1:3 and Colossians 1:16 would be lying when they say all things that are creations were made by Him and nothing that was created was brought forth without Him, as the first creation ever, "Time", wouldn't be made by Jesus. Neither would He be first in all creation, time would be.
In their silence, I ask "So in this state of timelessness that Jesus was supposedly created in, where exactly did God create and put Jesus if space didn't exist yet?"
This of course prompts more silence as all created beings, physical or spiritual, occupy and is limited by space. Before all of creation (Time and Space, and yes I know saying "before" creation would indicate time but bear with me here as we're trying to grasp a concept virtually impossible to grasp because of our created limitations), there was just God. God with no time and no space. God is all.
Therefore if Jesus was created before space and time, we have major contradictions happening and to try to explain away them would generally be giving it your best guess with no guidelines set in the Bible. Jesus must be God. Eternal (Non created) and without Space. "Before" creation, there was God. He was all and since Jesus was there, that makes Him.....Jesus.
If I'm wrong somewhere please don't hesitate to point this out. This is new for me and I've never seen anyone argue this way.