My first go through I just cheesed chapter 21 by only using Camilla and MU and flying from point to point to freeze the enemies. It took like 2 resets or so before RNG favored me enough to let me through. Of course, my Camilla in that runthrough was pretty strong (she was like level 16 Wyvern Lord at the time of that level), so it might not work so well if your Camilla isn’t up to snuff.
The second time through I had to reset twice, but I just did it more conventionally and basically wiped the map of faceless save for the ones that spawn at the bottom. I had Ophelia with a forged mjolnir so that she could one shot the golems provided she crit, and a Berserker Charlotte with a forged killer axe to accomplish the same thing. It’s a lot of hoping for RNG crits, but between the two of them I usually managed to avoid too many sticky situations involving the golems. Effie took like 1-2 damage from the faceless per hit, so I generally had her leading the charge and just having the rest of my dudes pick them off.
The fact that they all have savage blow is really what made it difficult for me, but fortunately Effie is so tanky that it barely mattered.
I’d suggest doing the paralogues unless you’re waiting to pass down certain skills to the child units.