After you lure the officers to the meeting point, defeat the opposing officer before he tells Nobunaga. After doing so head towards the opening gate after you hear from Hanzo. This will work when Yukimura says there is something suspicious about the gate.

-jay

you also need to complete shingens story mode first.

You will see a green circle heading up to shingen takeda after the ninja guy says
something suspicious, the ninja starts in the bottom right side and makes his way up.
catch him before he reaches shingen takeda.

patayin nyo si hanzo tapos ang bo-bobo nyo!!!

head to the main camp before he does