Aw man I remember chapter 19. *shudders*
My strategy: high speed units for normal stuff. Keep whatever healer you have alive, preferably paired up with a tanky unit
just in case... If the upstanding members of society put up an illusion, throw a Nosferatu mage (or whatever unit doesn't die in more than two hits on your end) in range of them and have everyone else back up. The upstanding members of society'll try to kill the mage. But with Nosferatu equipped, if your mage hits*, it will make their efforts for naught and then you can storm them with archers or wyverns or whatever hits these things because I forget.
Mr. Boss upstanding member of society at the end of the map is also just the WORST. I'd use Camilla for him if she can one-shot him reliably. Beware that he shifts between illusions for himself and his guards before you get in range. I also think he can one-shot a mage, but I may be mistaken. He's a pretty tough cookie... but since he's using a Beastone
+ he's practically a sitting duck after he attacks in enemy phase. Unfortunately he still has those two guards on the side of him... so they might complicate that part a bit.
That's just what I've done... but I'm really a Fire Emblem noob and shouldn't be typing up big paragraphs of what I consider to be a "strategy"... but I hope I helped if you haven't already beaten the chapter!

*I think Dark Mages get um... Heartseeker(?) at a certain level. I think it lowers enemy avo by 20, so that skill would REALLY help in getting hits in on those upstanding members of society.
Also was any of that good? Like I said, I'm a total FE noob, but I'm curious if anything of what I said made sense to a more seasoned FE player.