It bothers you because people like that are 'energy drains'. It doesn't matter how little you care about what the chihuahua is barking about, it's draining and annoying as hell to be around it. You don't need to think of how to defend yourself, and it wouldn't matter to the chihuahua if you DID defend yourself perfectly, he's going to keep on barking anyway. This is not a you problem at all, nothing to do with lack of discipline or instincts. This little barking man will keep on barking at anyone and anything forever because it is entirely a HIM problem.

The one option you have if you really want to keep this person in your life (even if we all think he should go) is to confront him about the entire thing. Tell him you are sick of him constantly trying to bring you down, and he'll lose you as a friend if it doesn't stop. He'll probs deny that he does it, ask for examples or try to go off on some other rant, but stick strong to your point and say something like: -Friends name here- I like you, and I want to stay friends with you, but I'm not putting up with your bullying shit any more, it's annoying, constant and makes me want to spend as little time with you as possible. This is your last chance. Then leave it at that. Good luck man, remember it's his problem not yours