Go through the Siege of Odawara Castle in Story mode under the Hard or Hell difficulty setting. You must defeat all the Hojo officers by yourself. The sequence is your decisions to make, but the following is recommended. You can start by razing any enemies you can see until the siege ramp is ready. Once it is ready, protect it. When you are beyond the walls, defeat the officer that guarding the cannons. Note: Do not kill the Reserve Captains as this will keep the allied officers busy. Then, defeat all three officers at south gate (the one with Shimazu). Ignore the storehouse sub-mission once you are done. Kotaro will send three more officers to your main camp. Go and defeat them. If you are fast enough, defeat three officers at main gate. Continue opening the gates by starting from the middle gate and again, defeat the two officers guarding that position. After that, proceed to north gate to defeat three officers there (one with Masamune Date). Go to the northwest area and defeat the two officers before Masamune does. Once done, make your way into the castle (the bomb corridor) and defeat the two officers guarding it. Again, go into the castle and immediately go to south or lower left area where the storehouse is located. Defeat all the officers there. You must hurry before the Shimazu army does. Then, proceed to the second floor in the castle and defeat Kotaro. A supply team will show up in the east of the castle walls. Intercept them, pick up the Coiled Viper weapon, then defeat the two Hojos at the third floor.