Weston,
If I were you, I would not try to "defend" baptism from a Presbyterian/Reformed perspective--no more than I would attempt to "defend" calvinism, if I were in your shoes.
I grew up a Presbyterian and a calvinist, before I even knew what those words were, or what they meant. The first time I felt a "challenge" from anything different, a young friend of mine gave me HalLinsey to read. I was fascinated by his apocalypticism, and I went and asked my dad what to make of it. He just asked me, "Does that sound like anything we've ever taught you? Would we have left out some important truth like that, if indeed it was true?" Dad never told me not to read it. He just taught me. He had confidence in the truth.
I went to a Baptist/Arminian institution for college. In my first semester, I was "challenged" on my particulars by my roommate. I knew what I believed, but I had never studied these things to defend them. I went back to my books. I refreshed my beliefs and confidence.
And the thing that amazed me the most was: I never got a chance to "defend" myself. My roommate never again even raised a challenge to me. I think it was attitudinal--maybe its "mythological" to think so, but I simply didn't view myself anymore as being surrounded by a hostile environment. Socially, I was completely inept, a basket-case. But theologically, I think I just knew I was an "alpha-male."
And I also didn't go looking for challenges. I didn't need to prove it to anyone. I can remember one other occasion in four years of college; someone expressed some surprise that I was a Presbyterian (Really, what are you doing at a Christian college?). I gave him a one or two sentence answer that defended my view of baptism, clearly based in Scripture. And that was that. He said, "Huh. I never heard anything like that before."
But--here's the meat--I don't think I was in any position to "defend" the doctrine of baptism, or even calvinism. I was still a "novice," though having grown up in a Presbyterian church. I wasn't being called to contend with all the Baptists that surrounded me. I needed to learn who these people were, to listen, and to gain sympathy for people who were different. While I reinforced my own confidence in Reformed doctrine.
Let your friends do the challenging. All you need to do is have a calm answer for them, whether positive or negative. You will learn what they believe are the "silver-bullets" and "nuclear-options" for the doctrines they hold. Their willingness to crusade, to persuade you that you are wrong and to win you back to something other than The Gospel, will expose what is truly important to them.
And in case you missed it, my point is that what you contend for with greatest vehemence will also prove what you think is truly important.