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To get Nene's 5th you'll have to basically just protect the siege ramp and get 400 K:o's within a time limit of 8 minutes or so. It's actually pretty simple... equip Matzukaze, Persian or preferly Moon. Do the first missions and make the siege ramp appear. Now, just basically ride around it and use Nene's super spicy horse moves. You'll be able to hold the enemy at bay, as they'll focus on the ramp and won't mind you running around it in circles.
Now, enter Odawara and start slaying people towards the north. Taking down the officer and guard captain outside the northern entrance of the keep is clever.
The weapon will appear now. NOTE! DO NOT RUN FOR THE WEAPON YET
Hideyoshi will now enter the keep, and an ambush will appear outside the main camp. Quickly clear away some of the enemies around Hideyoshi and defeat Kotaro. Now run north and defeat the enemies around the northmost garrison. Ride through the main camp and defeat the enemies who's procceeding from the south and west. The one in the north will be kept at bay by Mitsunari and his crew.
After this Mitsunari will move towards Hideyoshi and your other allies will charge through the Odawara main gate. Defeat the northern officer and get the weapon.

You can also run for the weapon straight away, but Hideyoshi or the Main camp sometimes struggles alot.

Recommendation:Switch Nene's fourth or fifth weapon element to lightning which is her best element. Remember to have two of the weapon before you do so