As you form your party to go fight. Push the select button and go to free battle. You can train as long as you want before the actual battle. If you defeat all enemies, you will show up back where formed you're party. Just keep doing that until you are ready for the actual battle.
Beat all training exercises in Free Time. They will have little gray swords on the top left corner of the picture. (You must win all training exercises if you wanna get bankai early.) Fail 1 and you don't get Bankai and you will have to wait till the end of the game.