Urjack
You have to go through mediation first, just be absolutely clear in mediation that you have done the work as required and are not going to back down.
Say only as much as is absolutely necessary, and as little as possible. Stay professional and calm. Do not waver from your line.
Ask here if there is anything that is not clear or makes you feel uncomfortable.
Mediation will push for a 50/50 split.
Do not agree to that.
Much of what I know about disputes I have learned from @TopRatedTroll. I only ever had one dispute. (which was a joke because I told the client I didn't want to continue working with her, and theoretically she was outside the dispute period, but someone ín support told her if she added a funded milestone she could then dispute.
But I remember thinking at every interaction "What would @TopRatedTroll do!?" and it worked.
I ended up being able to close the contract, keep my money and she didn't even leave poor feedback.