It probably should be in the format of a good argument, supported by valid premises. That would make arguing for or against free-will..... .... challenging, or impossible.
Thinking about the deadline approaching, you could do something easier. I haven't taken a philosophy study in a long time, but (maybe) it would be better arguing against something in the media, or a science news article or something. You could probably find a lot of good claims to argue against.
For example, there was one where research showed that plants have feelings. Or a study about dogs feeling uncomfortable when hugged from analysing MRI scans. That last claim seems really easy to refute; most humans are uncomfortable when hugged as well, and I doubt that the finding wasn't a really bad inference.
Not completely sure what I'm talking about, but my main point is finding something that's actually easy and doable, then do a good job on it. After all, it's probably just a demonstration of your understanding of the subject learned thus far, and how you are applying it.