how do you get yukimura sanada's 4th weapon?

my little brother said there is none

Yukimura does have a 4th weapon.u have to do certain tasks in Story mode on the Hard difficulty on Osaka Campaign.
his 4th weapon is called Dragon's Tail

Here's how you do it:
1. kill the officer in the castle
2. exit the castle and kill all the officers in the courtyard
3. destroy the cannon before it begins firing
4. kill Ina before she enters the castle
5. kill Masamune Date before he enters the castle

if u do all this correctly, a special item report will pop up and u have to break a black box on the 2nd floor of the castle on the same route Masamune was going