I tried every method mentioned here and none of them worked for me ... f* this Achievement.
Payed as Tokugawa, conquered everything - nothing happened from 1614 on. Played as Toyotomi, did nothing but helping Tokugawa - nothing happened from 1614 on.
I've no friggn clue why it doesn't work, I've easily spent 10 hours now just to get this Achievement, but despite playing every method several times, I never got the stupid trigger once! And yes, obviously I moved Sanada to the Toyotomi clan every time.