I didn't promote any of my units until chapter 16ish. Guard naginata Oboro paired with Silas or Rinkah was enough to survive.... After the damage reduction skill of sakura.
I had my spear fighter with guard naginata tank him at a choke point. Seal defense works wonders. Tank until guard gauge fills, attack, rinse and repeat. Burst him with magics when he gets low enough.
EDIT: based Oboro carried my team that chapter.
I've been playing on lunatic, and the growths are set in stone. I had to restart cuz the only unit that went over 14 strength was sakura. I even have a strength boon corrin with just 13 strength at level 20.
It feels bad. Fixed growths has me salty right now.
How do you all feel about the current growth system?
I'm personally having a bad time. I haven't finished any of the routes yet, but at level 20 the unit with the highest strength was sakura at 19. I ended up promoting her into that bow wielding healer.