Legendary Story- Tachibana Ginchiyo-Third Mission

I'm stuck on the one where you have to capture the three enemy castles to lure the enemy armies away from sieging your allies castle, which you have to hold for 16 turns. But it seems like I can't get to the other castles fast enough before the allied general gets killed. Also, that defense buff the enemy generals and some of their other units get is rough. I also lack any good general with siege ability so if that's part of the problem, for the moment I guess i'm screwed? I'm kinda clueless on how to do this.