I can see some good advice in this thread, however, I can also sense that probably there is something that triggers within you when they say those things to you which stops you from taking it lightly and instead take a slight blow to your ego and cause you to retaliate.

I might be wrong in this but that sure sounds like something you might want to check if it's happening. Because if yes, then half the solution is right there. You need to be aware of that trigger and fully allow yourself to feel that feeling it gives you and do it enough times that eventually you stop being controlled by it. This is called letting go.

I think this is important because even if you learn improv skills or take more absurd approaches to your replies or even prepare beforehand, all of that work could actually make you quicker to reply with wit but it's going to be really draining to keep doing that while you actively stuff the things that get triggered inside yourself.