Instructions
Preparation
Get yourself a good fourth weapon with at least two bonuses at +20 or better.
Make sure you have played through Magoichi's story mode at least once.
Power Magoichi up to the max or reset his stats--either one will make the stage easier. The former will give you more skills, while the latter will make the enemies easier to battle.
Run through the Defense of Echizen and try to complete all the missions. This will help prepare you for the real deal.
The Battle
Start Magoichi's Defense of Echizen on Hard difficulty.
Defeat Katsuie Shibata and Nobumori Sakuma in under two minutes.
Head over to Mitsuhide Akechi and perform the sniper mission. You must succeed.
Avoid Hanzo Hattori by running around the map until he gives up.
Head to the northeast and help your allies escape. Once all five of them escape, a supply unit will appear in the center of the map.
Chase down and defeat the Supply Captain for the weapon.
Finish the stage.