What can I do if the defendant is threatening to sue me for his costs, damages and more?

I know what I'd do and it doesn't have anything to do with the law. Which is why I don't have to deal with punks harassing me but from a legal standpoint, there's not much you can do about the e-mail unless it was physically threatening and / or a violation of a restraining / no contact order. As to the chances of him successfully suing you - depends on the facts. To me this whole thing sounds like a bunch of idiots who have no idea how handle business proper and like to run their mouths reckless while hiding behind the court like it was their mother's skirt. So odds are, when he says he's going to sue you - it is most likely more idle idiotness. However, if you accused him of a crime and he didn't do it - you could be held liable for slander and / or libel per se and quite frankly, if you did something idiotic like that - you deserve to get a spanking in the pocket book. I am curious as to why you refer to him as he for most of your question then he/she and finally as 'this person'? Do you not know who is harassing you? Further, you say 'oh, i just want this over'. Really? You had the man(?) charged with assault, ultimately convicted on some charges - are you surprised that he would now want some pay back? What can you do? Count yourself lucky that he is silly idiot who is resorting to lame e-mails instead of some one with the stones to burn your house down with you and your family in it.