In-game reset:
Hold Start + Select during game play.
Unlock all characters:
Have a saved file from Samurai Warriors on your hard drive or on your
memory card
Shield Wall:
Play the Battle Of Komaki-Nagakute with Honda Tadakatsu.
Win 1st place in every missions, after beat the supply team.
War Baton:
Play any versus mode option minimum 35 times.
Source? Here it is.