WARRIORS OROCHI 3 Ultimate

A simple achievement. Once you start gauntlet mode, you will need to pick five characters for your team. All the other characters besides those will be locked even if you already unlocked them in the story. To unlock them for use in the gauntlet mode, you will need to defeat them in battle, after which they have a chance of dropping their "Sworn Ally" item. Once you've picked up the said item, after you finish the battle and are back in the camp, simply talk to the mystic, choose the summon officer option and unlock any officers you have available and the achievement is yours.