I actually have a considerably higher test verbal IQ than mathematical and have perfect verbal section test scores to show it. However, I used to really suck at debates because I was naïve and unpracticed.
I went through a few years of spending too much time on reddit, though, and now I can wallop most average people. The most important thing BY FAR is to immediately establish definitions and frame the debate. You force your opponent to operate in the tiny amount of air space that you've given them, and constrain them to talk about things that you've chosen so you probably know more about them.