Instructions
Preparation
Get the Matsukaze! As per usual, you will need the legendary Matsukaze horse. Only this time, it is essential instead of recommended.
Get a good 4th weapon. You will need a Muramasa with at least 2 stat bonuses of +20 or better.
Bring backup. A companion fighter may not be necessary for this mission, but it certainly won't hurt.
Reset if you need to. If you have tried this before and had trouble, reset Nobunaga's (and his companion's) stats. This will reduce not only your level, but the level of your enemies, since it is based on the average level of the player(s).
Make sure you have played Nobunaga's story mode through at least once.
The Battle
Begin Nobunaga's Battle of Okehazama on Hard difficulty.
Defeat both Defense Captains that appear for your first mission.
Defeat the sub-generals inside the gates if you wish. Not necessary, but it helps.
Ignore Hideyoshi. As cruel as it may be, when your general Hideyoshi cries for help, ignore him! After about 3 to 5 minutes, he will be defeated and the Imagawa will receive some support.
Destroy the Imagawa reinforcements except Ieyasu Tokugawa and one other elite general.
Head towards the main entrance to the Imagawa camp.
Hanzo Hattori will appear in the far southeast of the map and say that he must kill Imagawa.
Rush to the Imagawa camp! You MUST beat Hanzo Hattori to the Imagawa and launch your attack before he can make it there. This is where the Matsukaze saddle comes into play.
After Hanzo retreats, the Supply Team will show up where Hideyoshi first appeared. Rush there and defeat the Supply Captain for the weapon.
Go back and stomp Imagawa into the ground. Congratulations! You now have Nobunaga's Orochi sword!