I could be wrong but it sounds like you went to a Mormon chapel, maybe from curiosity, or a desire to find out what they believe; decided you didn't agree with it, left, read the Bible and became a Christian.
But that now you're going back - maybe in the hope that someone will say something that you can question/contradict, which may, in turn, get others there to question their faith.
You may have the right motives, but I don't feel it is very wise and may even be dangerous.
You are on your own, surrounded by Mormon followers and teachers. Does anyone from your own church know where you are and what you are doing? If they did, at the very least, they could pray for you while you are doing this - though it's likely they'd tell you to get out of there.
Why not spend your time going to Bible studies/lectures/doing courses/meditating and building up your own faith? Then, a bit further down the line, with prayer and Christian support, set up some kind of joint meeting/public talk on neutral territory. Once there, talk to them and find out what they are saying, listen to them and above all, SHOW love and acceptance of them as people. Share your testimony, if appropriate - say that you once attended their meetings, read Scripture for yourself and then left their church. I am pretty sure they'd question YOU to find out more and try to win you back; if they do, at least you have other Christians around you.
As a teenager I spent hours talking to, and arguing with, JWs. Moonies etc - though rarely Mormons, admittedly. I once went to a Moonie commune with the specific idea of arguing and converting. It occurred to me, while I was there, that no one else knew where I was - if they had worn me down, confused me, then bombarded me with kindness and, their version of, Scripture, I could have ended up going back/wanting to stay and convert. For all I knew at the time, they could have locked me in and not let me leave. They didn't, and obviously I never went back.
I'm not saying Mormons are like that - but why put yourself in that position?
IF you are called by God to do this work of witnessing to other faiths; do it properly.
i) a good grounding in Scripture and your faith
ii) Certainty that this is God's will for you; the strength and filling of the Holy Spirit.
iii) support - definitely prayer, preferably also the presence of 1 or 2 Christian friends. And inform your Minister/Pastor what you are doing.
iv) Love, love and more love - listen, accept and respect them, don't itch to argue and convert.
v) Leave results up to God; share your testimony, faith and relevant Scriptures, debate calmly and politely, pray as you are doing so - then leave.