How do you produce a finisher in Super Smash Bros Brawl?

Answer 1

I assume you mean a "final smash." This can only be obtained in 2 ways. The first is to break a Smash Ball by attacking it and then using ones standard special attack ("B" on a Gamecube controller). The other is to be losing by a significant number of lives in a battle, in which case you may spawn with a Smash Ball already in your possession (though this will not happen every life).

Hope this helps!