Filing a brief should be optional, unless the judge specifically ordered you to file one.
The brief should be on numbered pleading paper with a case caption on it. Alternatively, you can use a Judicial Council form as your caption page. I am thinking that if you are trying to tell the court about some irregularity in the mediation, you may want to submit a declaration under penalty of perjury. See link for form below.
However, as far as the contents of the brief or declaration, you haven't supplied sufficient information for us to give you any advice.
Filing a response may not be appropriate or wise in this situation. If your ex's lawyer wants to addrss some issues, it may then be appropriate to respond to that. but right now it sounds like you should quit while you're ahead.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.