I've already posted this on multiple occasions.
The best option is the BrawlPlusTweaker (search in the workshop for it, its made by Shortfuse). It is coded in the .NET framework which is compatible on the Mac by downloading the Mono Framework (google search: Mono framework)
After installing that and downloading the tweaker, you can launch the Tweaker by opening up Terminal (in your utilities folder), and using the "cd insertdirectoryhere" to change directories until your where you have your tweaker folder. Then launch the application using "mono brawlplustweaker.exe"
You'll have to resize the window once, but it works perfectly.