hi i am a new member please cut me some slack in this question how do i unlock the
last gaiden of the army

Sorry make you wait so long. I hope you still there when I reply your request. In order to unlock the last gaiden, you need to perform a special conditions in 7th stage.
-SHU : In the Battle of Edo Castle, when the castle/main keep is burned, you and your allies must escape from castle within 3 minutes.
-WEI : Defeat Da Ji one minute from her defection.
-WU : Sun Jian and Sun Quan must join your army within 3 minutes (they'll join your army after the scene).
Tips for Wu : You don't need to defeat every officers around them, or it'll be hard to finish it in 3 minutes. I suggest you reduce their life and let Sun Jian or Sun Quan kill them.
-SAMURAI : Score 1000 KO.
Tips for Samurai : The enemies will respawn more in central garrison (Keiji Maeda's starting place).

Hope this can help. If you have further questions, just ask me.