It might be just me but i find the game to be atrociously grindy.
Ennemy forces are just growing way faster than you can sustain yours and by turn 50 i'm facing armies of 1000 when i can barely field 500 ones...
Lots of choices but the core mechanic seems broken.
Ironically, grinding to get your units to be numerically better is the exact wrong way to play Sengoku, which is very counter-intuitive. AI scales based off your own unit numbers (for the most part), so it can actually pay to delete all of the soldiers in a poor combat unit and just have it do various non-battle tasks instead of fielding a bunch of 500 man, but poor stat, units. The better idea is to train the stats of good units (ie, by go into dungeons to get treasures, do the H scenes and various tasks that pop up in the provinces), and to get the combat books from the province tax events. You do want your big units to have large numbers (at the beginning of the game, Rance, Ranmaru, and Katsuie will do you fine), but having 500~ soldiers or so is more than good enough for the vast majority of early game battles. Anything more than that gets diminishing returns, imho.
Some of the enemy nations are also just more potent than others. Generally you want to clean up the weakling nations while poaching the best commanders from them; there's a godlike Ashigaru in the tanuki land (Tokugawa) that can be taken with reasonable ease, and the priestess in the mountains (Miko Union) is also incredibly good. The bowgirl from Ashikaga is also great, and I'd take out Uesugi as soon as you can for the best samurai and the 2nd best tactician in the game. Uesugi, on paper, is a difficult nation to take out, but there's a special event when you field Rance against the head honcho that makes it far easier. A general early game strategy that won't do you wrong is Hara > Ashikaga > Azai/Tokugawa > Miko > Uesugi. After that, you can do pretty much whatever, though I'd prioritize the non-gourd nations first.
Also, don't sleep on tactician buffs. Akechi is bad, so it's reasonable to think they're a poor unit type early in the game, but the buffs higher intelligence ones can give your units are insanely potent, I think it's around 40% more damage and defense or something if they get the up arrow on their unit card? I don't have the numbers immediately in front of me, but the wiki should have all that info if you care enough to look. Lastly, use Rance's satisfaction to call in foreign reinforcements. The continent units are all pretty great, though I'd go down the Zeth path before Leazas to get Urza. She's the best unit in the game by far (for both how early you can get her, and for her turn-deleting abilities), and can easily turn the tide all by herself.