Well to build what should be insecurities into strengths self reflect on what your strengths and weaknesses are. Accept them, be comfortable with them, make fun of yourself with them and openly make fun if them to other people. That way you've heard it all they won't phase you a bit. So you don't shake or weaken.
Its hard but once you conquer this part you can now begin to look for insecurities in others. You pick up what makes other people nervous and what their general insecurities are. Focus on those, devise your jokes, store them in your mind
When the roast session breaks out attack. Attack fast, use everything you know about them, use your surroundings use pop culture references its all to keep them unsure where you're going to go next and while they're thinking about that and where you're going to go they're slowing up and everything they're hitting you with doesn't phase you cause you've already roasted yourself dozens of times.
