Hey there,
Voice of experience from the State's side of things. You probably will get a subpoena, maybe just shortly before the trial. Sometimes trial schedules are in flux--there might be other cases scheduled as well, and it might not be clear that your case is a "go" until shortly before trial. Those are questions for the victim advocate. If I were you, I'd want a subpoena just so it was clear you weren't there "out to get him". You are there as a legitimate witness in a criminal case.
Don't even think about pleading the Fifth. It will make you look ridiculous. You have no legitimate expectation of being prosecuted for anything, so you don't have a basis for claiming the privilege.
You will be just fine. You don't have to be anything but a neutral witness--you don't have to be reluctant or champing at the bit. You're a witness, you show up, you tell the truth to the best of your ability, end of story.
Oops, accidentally posted too soon.
Other advice--ask to meet with the prosecutor in advance (he/she will probably want to prep you anyway). Ask what he or she intends to ask you, so you will be prepared. Answer the defense attorney's questions in the same helpful spirit that you answered the prosecutor's questions. If there is any "fixing" of your answers that need to be cleared up (due to a misleading impression the defense attorney is trying to create) the prosecutor will have a chance to ask additional questions on re-direct.
Don't volunteer information you aren't asked for (even if you think it will be helpful). That will only cause problems. Listen to each question, make sure you understand it (and ask for clarification if you don't), and answer as honestly as you can.
That's it--the rest is for the lawyers and judge. You can breathe easy, knowing you have done the right thing.
One other tip--don't look at him while you're testifying. Look at the lawyer who is asking the questions.
This is good practice at letting go of the outcome. It isn't your job to make sure he's found guilty, it isn't your job to make the event any worse or better than it was. It also isn't your job to say whether he did something "on purpose." If the defense attorney were to ask you that, the prosecutor should object. You can't read his mind. You say what he did, and the judge is the one who will have to conclude whether he acted intentionally or not.