Nice and easy one for this, Once you have finished your second mission in the Story mode come out of it and go into Musou Battlefields. This is a personal edit mode for battles. All i did for this was choose the second mission which for me was Battle Of Ueda Castle. I changed one little bit of information in the battle, (whether this is needed to unlock the achievement i do not know). Once you run through this mission the achievement will pop at the end of the battle.