Here's the Enotes definition:
A polemic (pronounced /pəˈlɛmɪk/) is an argument or controversy made against one opinion, doctrine, or system, in support of another, or a person who engages in such disputes. A polemic usually addresses serious matters of religious, philosophical, political, or scientificimportance. A polemic is often written specifically to dispute or refute a position or theory that is widely accepted. The word is derived from the Greek word polemikos (πολεμικός), which means "warlike," "hostile".
It is a strong written attack on someone (Ad Hominem) or something (a controversial topic). It's very similar to a satire and a rant, in which you are presenting mainly your side of the argument, not necessarily your opponent's. It's highly personal and emotional writing, does not employ the stuffy elements of a traditional persuasive paper. In short, it is full of pathos in an effort to make your audience feel the same anger and disgust that you feel.
In terms of topic, you will want to think big. You will want to take on a traditionally accepted viewpoint or, if it's against a person, someone who is a kind of media darling. Do not begin inductively. You will need state your opinions early and often--no holds barred.