SEC542 is a very basic course, which prepares you to do the GWAPT exam, where I was given a test exam a couple of years ago. I had never studied the course, yet I passed without knowing about SOAP and a few other things that you hardly, ever see when conducting a penetration test. (I have tested a few big lists of web services, and yes it was boring, but at least I gained some experience during that, such as it's rarely any critical bugs you find, it's often just user account related problems, often rated as low or medium risk. Meaning it's just "filler space" in a report.)
Anyway, yes you can do it. I'm a hacker, and yes I knew how to write secure PHP code when I took the exam attempt (I wasn't taking the actual certification). To me, it was extremely easy, but then again I am already a hacker so of course it's easy when I specialize in web application security.
Having previously taken GPEN (and passed, and become a mentor), I knew how the exam layout would be, how the questions would be phrased (i.e. in a weird way that doesn't make sense until you understand "their language"), and of course how much you had to think about each question. Because when I did my first GPEN exam attempt (test exam, not actual exam) without any study, I passed as well without any study, but I wanted to increase my score, so I studied a bit. Anyway during that test exam, there were a lot of questions where I was thinking, is this a trick question? This is way too easy to be included in this type of exam that proves your skill, but they weren't.
Sometimes the questions can be a bit strangely phrased, so you will have to think for a while about what they actually mean. But it's nothing compared to some custom CEH courseware I went through for fun, while I was working as tech support and wondered how hard CEH would be. Not because I want the cert though, just want to see how hard / easy it would be to pass, plus it was free at my previous job. (The custom courseware.)
I'm an InterN0T'er