I have tried the tips people have given on this forum for this campaign and though it may work for some it doesn't for others. 5 minutes research later and I may have found a sure fire way which I am now working through.
Before I begin here is a quick history lesson of the actual events that samurai Warriors portrays.
Battle of Sekigahara
Sekigahara was the most decisive battle in Japanese history. Fought against the ritualised and colourful backdrop of Samurai life, it was the culmination of a long-standing power struggle between Tokugawa Ieyasu and Hashiba Hideyoshi, two of the most powerful men in Japan. Armies of the two sides met on the plain of Sekigahara on 21 October 1600, in thick fog and deep mud. By the end of the day 40,000 heads had been taken and Ieyasu was master of Japan. Within three years the Emperor would grant him the title he sought – Shogun.
The similarities between the game and RL are too close to call. When you begin the map Ieyasu is ranked 2 - only 1 step away from Shogun. The battle itself in the game begins in summer/fall 1600 exactly the same as RL. So that got me thinking - if the game portrays the actual events as they happen then what exactly did happen at Osaka in 1614-1615.
Battle of Osaka
In 1614, Osaka Castle was Japan's greatest fortification, measuring approximately 2 miles in length with double circuits of walls, 100 feet high. It was guarded by 100,000 samurai, loyal to their master: the head of the Toyotomi clan, Toyotomi Hideyori. The castle was seemingly impenetrable, however the ruling shogun of the age, Tokugawa Ieyasu, was determined to destroy this one last threat
to his position as Japan's ultimate ruler.
So by my reckoning, if the game is staying true to life all you have to do in order to achieve this is to take over the map as Ieyasu except
for the 2 areas covered by Toyatomi by 1614 then the battle will begin. For those of you who have played as Toyatomi, you know they are allied to everyone so there is no worry of them being defeated. Also, as pointed out and also verified in RL, Yukimura Sanada was one of the generals who stood against Ieyasu at Osaka so he will need to be moved to the Toyatomi to be protected for the final battle.
Simplified - move Sanada to Toyatomi, select Ieyasu (for reasons I will point out in a second), open a can of whoop-ass on everybody on the map then have a quick coffee and prepare for the final battle.
I'm currently working through this way to verify it does work and will post my results once I've done.
Finally, the reason I say to choose Ieyasu rather than Toyatomi and the reason that following the guide hasn't worked for some of you is because in order for Ieyasu to trigger the event, he needs to be next to the Toyatomi in order to attack. I'm sorry, but letting the AI do that is far too random especially if you are having to sit patiently for 15 game years
For those lucky enough to watch Ieyasu take surrounding land by 1614 you will have initiated the final battle, for those of you like me who after 2 or 3 attempts couldn't work it out then now you know
P.S I'm also looking up to see who Ieyasu's main Generals were for that period as that may help with the 4 generals achievement so many people are desperate to get. Could be that the achievement is not random afterall but infact following RL again.