He is not going to annihilate you.
Firstly, coming from a training course I did years ago on negotiation: if you go in believing he has all the power - you relinquish your power.
You have things he wants. Mostly these relate to control.
Go in hoping for the best - that this will give you what you need. If he won't entertain it, then there's nothing you can do.
Mediators are trained professionals. They will facilitate this properly - it's their professional pride at stake after all.
If he wants to dictate terms, he won't be allowed to.
And shuttle mediation is very limiting. You don't have to deal with him directly. You do have to turn up, smile, state your case calmly and then go home go about your day, knowing that you've completed another step to independence.
Good luck Karen - you don't need it, but anything extra is a bonus!