From the main menu select 'Fight' and then select 'AI Battle'.
Next pick any 3 characters as a defending team and then press A to save the new team. After this select 'Pick a Battle' and pick any team to VS from the menu. Next pick any 3 characters as a attacking team and then press A to confirm the attacking team.
When the battle starts press Select twice to speed up the gameplay to 4x the normal speed. When the battles are over press A to return to the 'Pick a Battle' and repeat the process 9 more times to unlock the achievements / trophies.