A little trick that I used to use when the AI targets one of generals or leader, move them to a province with loads of shinobis in it. It reduces the chance of success considerably, with 20+ shinobis it can almost make it impossible for them to succeed. It can make the later part of the game enjoyable compared to the Benny Hill chases that could go on with the Geishas chasing my leader while numerous assassins & Geishas chase the AI's geishas.