Step1
To begin, import data from the original Samurai Warriors if you haven't done so already. Set the difficulty to "Chaos", or if you have purchased the special bonus available from the options menu that allows you to obtain Ultimate Weapons on Hard difficulty, you may set it to Hard instead.
For help on importing data or setting the difficulty:
http://www.ehow.com/how_2108980_samurai-warriors-samurai-warriorsxtreme-legends.html
http://www.ehow.com/how_2108979_set-difficulty-samurai-warriorsxtreme-legends.html
Step2
Begin "Story Mode" - final weapons must be earned in Story Mode, not Free Mode, for all characters. Play as a created officer using a spear and begin the Battle of Kawanakajima stage.
Step3
Equip Tengu Sandals or, better yet, a good saddle, such as the Matsukaze Saddle. You'll have some ground to cover. You'll need to keep all of your general's alive in this stage, so if you have trouble doing that, consider the Battle Dru as well.
To get the Matsukaze Saddle, see:
http://www.ehow.com/how_2109786_matsukaze-saddle-samurai-warriors.html
To get the Arabian Saddle, see:
http://www.ehow.com/how_2109784_arabian-saddle-samurai-warriors.html
To get the Battle Drum, see:
http://www.ehow.com/how_2109783_battle-drum-samurai-warriors.html
Step4
You'll begin the stage in Hachimanbara, and will get a mission to attack the Uesugi advance force. Two enemy generals will be highlighted, but you only need to defeat one to clear the mission. Don't worry about clearing, the mission yet, though; first you'll want to secure Hachimanbara so your allies don't die.
First, head for Kagetsuna Naoe and defeat him, as well as the Reserve Captain near the Takeda camp that you will encounter along the way.
Step5
Taking care to avoid killing Kageie Kakizaki, turn south and defeat Yoshikiyo Murakami, and if you like, his officer Takahiro Kitajo. Seal the enemy stronghold near him as well.
Step6
Now that you've taken out a couple of Uesugi generals, head back to Kageie Kakizaki and defeat him (if you proceed any further south, you'll fail the mission). With Kageie down, head south toward Katsunaga Irobe.
Step7
At some point, a fog will descend on Hachimanbara, restricting visibility. When this happens you should be near the river that runs through the center of the map. Use this river as a reference, and after defeating Katsunaga Irobe, head east along the river to eliminate Shigenaga Honjo.
Step8
With all Uesugi officers in Hachimanbara defeated, keeping heading alongside the river until you see the bridge leading out of the main plains. Fight your way to Yukimura and begin assisting him as directed by the mission.
Step9
During your journey to the Uesugi main camp with Yukimura, you will bee ambushed from behind by Uesugi general Yataro Onikojima. Defeat his force to the rear and Masashige Jojo to the front to secure your path and keep moving.
Step10
Keep circling around to the southwest of Hachimanbara. Eliminate the two Uesugi generals Kagetsugu Amakasu and Kagenobu Uesugi. If you've done everything as outlined, there should be no more Uesugi generals left, since Kenshin will not be on the map at this point.
Step11
Once Yukimura's path the the blue X is clear of all enemies, scoot on back to the now empty Uesugi base camp atop Mt. Saijyo. Once Yukimura reaches his destination, wait for the ensuing dialogue to take place. Once Kenshin appears and you get the mission to defeat him, the supply team will show up from the stronghold in his base camp.
Step12
Collect your weapon, then chase down Kenshin and defeat him to end the stage. The Otegine is now yours!
Note that all spear weapons are accessible by any created officer who uses a spear. That is, you need not obtain this weapon multiple times for multiple characters.