Instructions
Preparation
Equip Mitsuhide with largely offensive items such as a fast horse, Vixen's Tail, Power Bracer and anything else that will make his enemies scream in terror.
Make sure he has a good fourth level weapon, preferably with a lightning element.
Bring a friend. This is much easier with a helping hand.
Make sure you have played through his Story mode first.
Start the Battle of Yamazaki on Hard mode. There is no need to make it more difficult.
If you try and fail repeatedly, reset both Mitsuhide's and your friend's stats. This will make it easier.
The Battle
Clear Tennozan by defeating the three guard captains.
When a request for support from your allies to the south shows up, go help them.
Rush back to Tennozan and clear it of enemies before Keiji Maeda charges or enemy reinforcements show up. This will happen around five minutes into the battle.
Do not let your guard captains at Tennozan die. If one of them dies, you must restart and try again. Have your friend stay at Tennozan and keep the captains alive.
Kill the supply captain. He will show up near the enemy commander. Kill him quickly before he gets away.
Finish the stage and enjoy your new weapon.