This is Shitaragahara, the part of Nagashino that everyone knows it for. Which means we're gonna see a looooot of our friends die today. Masayuki starts on his own, sealed off from the rest of the army, dealing with a few enemies and Hanzo's ambushes as the Oda face off against the Takeda elsewhere. By the time you're free, everything's gone pretty bad.
The Oda officers that defected to you, will now defect back and attack officers around them. Tadakatsu and others will rush Katsuyori, and your own officers will continue to rush the riflemen and constantly get themselves killed in an attempt to hold off the enemy for Katsuyori to begin his escape.
After you reach Katsuyori, he will start to escape but a ton of officers will block his path. Meanwhile, Masateru will attempt to charge Nobunaga and asks you to join. If you can reach Nobunaga in time, you will win the stage but you can absolutely help Katsuyori first just to ensure he won't get himself killed.