I don't really know what you mean by roasting, but if someone mocks you, says some disparaging comment, whatever, you don't have to have some kind of clever comeback. You can just stare at them for a couple seconds and say "Really?", say "Wow, you're an asshole", "Real mature" and walk away. Keep the focus on them being rude, don't fire back yourself. Don't get into an argument. Just make them acknowledge how they're being dicks. Also, if it's a friend who's doing this, tell them to stop, if they say "it's just a joke" tell them you don't find it funny and again be firm telling them to stop. If they continue, they're not your friends and you should stop associating with them.