Its been awhile since I had a similar problem like this, but it might be the same exact thing.
Before starting a match. Try selecting each character on the characters selection screen until you can find one for which the game freezes. Then, restart the wii and select that character again. If it freezes again, then the problem might because of the Fit*****.pac files found in the "fighter" folder of the SD card.
Go to the character's corresponding folder within the "fighter" folder and remove the files of the character that froze the game from the SD card. With the contents removed, try to reselect that character and see of it freezes the game again.
Files with the name exactly as "Fitike.pac", "Fitkoopa.pac", "Fitpikmin.pac", etc. hold information regarding attacks and hitboxes. If your playing a version of Brawl+, its likely that this file was edited poorly and oversized. In other words, it takes up more "bytes" than it should and thus freezes the game. That happens a lot with other files as well that certain edits make the file sizes too big and will freeze the game.
