I have just spent the last 16 hours trying to get this based on tips and stuff. Had sumin like 5 attempts and its just not worked but i have finally got it now .
I wanna make it clear exactly what i did so no one gets confused and it works first time for anyone else .
Put yukimura in in toyotomi and played as tokugawa as normal.
Invade the 3 flefs to your left and make an alliance with toyotomi from now on. (i say the 3 to your left because you need to be next to toyotomi to trigger the fight)
Dont take any more places than the 4 you shud now have and dont let toyotomi take any either.
also its important toyotomi doesnt invade anything because he might assign yukimura to a different area which is what messed it up for me.
that should be all you need to do just protect both tokugawa and toyotomi and stop him invading then you will be fine. just hope this saves at least one person some time lol