Played every path on Hard, starting Lunatic, and have just realized how to cheat the entire game. [Extremely minor spoilers for all paths]

Prison.

Awakening's Lunatic Mode was notorious for being downright unfair in terms of what many consider pure "fake difficulty", throwing out stats, skills and weapons units sure as hell shouldn't have and overloading the enemy count immensely in a way that generally makes it impossible to clear.

Fates does the same - seemingly toned down, if Hoshido Lunatic ch13 and down is anything to go by - but the Prison mechanic allows you to cheat the absolute fuck out of everything.

Ch13 has a General up in the top left - level one, with a steel lance and a javelin on them. Bait them away from the other enemies, throw Orochi (the only unit with the ability to capture as far as I know - Zero takes this role on in Conquest and both hold it in Revelation) on that immediately, Capture, pray Orochi isn't murderized in the process, and then beat the chapter. The result should be a General with decent stats sitting in your prison, waiting to be persuaded or bribed.

Either visit actual shittons of castles for resources, or play low-level grinding maps so that you can repeatedly persuade, or even just wait and persuade ever so often, and you now have a completely free General before the majority of your units have hit level 20 or promoted at all.

Enjoy life as a general carries you through multiple chapters