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-warriors.html
Step2
Begin "Story Mode" - final weapons must be earned in Story Mode, not Free Mode, for all characters. Play as Mitsuhide Akechi and begin the Battle of Yamasaki stage on the lower branch of Mitsuhide's Tale. Make sure you are playing on the upper branch, against Nobunaga Oda, not on the upper branch against Hideyoshi Hashiba.
Step3
Equip your standard items for this battle. You may want to consider throwing in a battle drum however, since all your allies must survive until the weapon appears.
To get the Battle Drum, see:
http://www.ehow.com/how_2109783_battle-drum-samurai-warriors.html
Step4
As you begin the stage, you'll be assaulted from both sides of the mountain summit. Head west and defeat Kazumasu Takigawa right away.
Step5
Once he is defeated, return to the summit and attack Narimasa Sassa, who will attack the mountain's east gate. Defeat him and return once again to the summit.
Step6
Hideyoshi Hashiba will arrive to reinforce the Oda roughly 5 minutes into the stage. To occupy your time until he appears, eliminate Katsuie Shibata in the north, then defeat Nobumori Sakuma and Toshiie Maeda in the southern slopes, and seal the nearby strongholds. Then, if Hideyoshi still hasn't appeared, go east and defeat Nobutada Oda and seal that stronghold as well.
Step7
Once Hideyoshi arrives on the scene, drop whatever you're doing and defeat him. You can return to clearing out enemies once he has been stopped.
Step8
Once Hideyoshi is down, defeat all enemy officers except Ranmaru Mori and Nobunaga Oda himself. Also seal any strongholds in the south and east.
Basically, finish what you started in Step 6. After you're done, move near Ranmaru to start a mission.
Step9
Check your generals to make sure all 9 are alive. If they are, let Ranmaru attack you for about a minute without killing him or running away. Keep it up until he joins you.
Step10
Head for the cave, where there should be a blue X. There are 5 "chambers" in the cave; go towards the blue X first, which will be in the central chamber. Approach it from Mt. Ten Oh, going down some stairs; make sure the spy leaders that will appear don't run past you into the mountain or you'll fail the mission.
Step11
After you've cleared away the first ambush party, go to each of the other 4 chambers. For each chamber, move around the center until an enemy ambush party appears, and defeat all of them. Do this for all 5 caves, and seal the strongholds inside as well.
Make sure that no squad leaders reach the mountain summit, or you'll fail the mission. Also make sure that for each of the 5 ambush parties, you don't move on until it says "ambush party eliminated!"; search the area for spy leaders if you don't get this message.
Step12
If all 10 (counting yourself and Ranmaru) of your generals are still alive, the supply team should appear in the southeast and head west. If they don't, make sure all enemy officers except Nobunaga have been defeated, and it will appear.
Step13
Track down the supply team and defeat the captain for the weapon.
Step14
Defeat Nobunaga Oda to end the stage. You now own the Rasetsu!