Legendary difficulty - dealing with late game hordes?

[Maltz]

If you are not opposed to the most powerful exploit on Legendary: Low-level agents are not a problem if you are determined to find the right random seed to make them lucky. Find any castle to biege with anything (like a Light Cavalry). Every time you choose whether to assault or continue to siege, there is a new autosave generated.

(1) Generate new autosave.
(2) Agent action. If succeed -> (3a). If fail -> (3b)
(3a) Generate new autosave. Backup the autosave in the save folder.
(3b) Load autosave generated in (1), generate a new autosave.

After you are done you can break the siege and pull the unit back, or play chase on the field. Light cavalry is perfect for this.

Before Realm Divide, it would be better to make one powerful ally (such as Uesugi) by gift/alliance/hostage and pull them out of their alliance with Ikko-Ikki.

Note: Low level monks can't recite rebellion, but it is very easy to take them to level 2/3 by army demoralization.