If you are a Christian and plan on witnessing to Catholics, my advice to your is to KEEP IT SIMPLE at first. Do not delve immediately into mechanics of Scripture, bouncing around trying to refute every single doctrine of the RCC.
The simplest starting refutation is how one is saved. Ask them to provide you how one must be saved or do they expect to get into heaven. And then provide the Gospel truth. Don’t bring in Marian doctrines, purgatory, false RCC traditions and don’t allow them to either. Keep the focus on how one must be saved. Provide the them the truth from Scripture, and keep the conversation simple.
Remember, Catholics are NOT studying Scripture in their faith. Converts from a protestant church will have a grounding in Scripture, but mainly try to use it only as a defense of the RCC doctrines. Though it is not a requirement for Catholics to be completely intelligent when it comes to Scripture, nor is there any accountability for this from the RCC.
So with the majority of Catholics, going to systematic theology from the get go or using words such as imputation is not going to register with them. They may nod their head in agreement, or most of the time they will try using a RCC talking point they memorized, but most of the time they really have no clue what in the world you are talking about if you get into Scriptural mechanics with them. They don’t want to APPEAR to look stupid, so they will try their best to play along with you. Avoid this with them, your teaching to them would be like a 5 year old trying to learn quantum mechanics in a day. I am not being mean here, it is the truth for the majority of Catholics in society.
Keep it SIMPLE at first. Why Jesus died for us, what salvation is, who Jesus is, and why faith in Him is the only way. They think they know who Jesus is for example, but it is the RCC version, not the Jesus of Scripture. Explain His deity, why His existence was more than a birth by Mary in Bethlehem. They view Jesus in just 3 basic ways. One, as an infant or child of Mary, two as a good teacher, and three as one who is perpetually dying on the cross for sins in their Eucharist, a Jesus encapsulated in a wafer and some wine. They need to be taught that He is and was MUCH MUCH more than these 3 basic views. Also they need to be taught WHY Jesus would leave His eternal throne and become incarnated. To a nominal Catholic, it was to prop up Mary, to glorify her as His mother, and that wafer (host) the priest holds up during a Mass.
In this keeping it simple approach, KEEP THEM FOCUSED. When there is something they do not like hearing, expect them to try and change and divert the conversation onto you or something else. Let’s face it, if they don’t agree with the basic Gospel, anything else you try to refute will be useless. So try and not move onto another doctrine or falsity, which is tempting to do, and they are hoping you will. The goal is to keep them off the merry go round.
They key here in witnessing is EDUCATION. Educate yourself on what Catholics believe, and why they believe it, focus on one or two things and become an expert on them. If you can convince a Catholic of the falsity, you accomplished a major duty! The crack in the wall will lead to MANY other cracks. You will just waste time on trying to convince them immediately off the get go ALL the falsities they currently are following. Try one at a time, if that first building block doesn’t go well, move along. Or when you come back to that same Catholic, try it again, stick with that simplicity until they begin to understand before moving along.