I'm honestly really tempted to skip saving Effie and Arthur.
They were literally a corner space away from each other. Just out of range of some Wyvern Riders. Ryoma/Rinkah Replica and Takumi/Robin were keeping the endless Malig Knights that come from the bottom of the map at bay, being ready to one round them right at their spawn point. Hinoka was keeping the Dark Knights that spawn from the top right of the map trapped in a hallway they spawn in. She doesn't one round them, but she is completely invincible to them even when they do a Dual Attack so she's fine. Everyone else is just sort of hovering near Effie and Arthur, mainly so I can have them ready to slaughter the Wyvern Riders on my next turn, particularly Marth and Lucina since Falchion/Parallel Falchion are effective against dragons, and Wyvern Riders count as dragons. Oh, and since a good amount of IRL time passed since my last play session, the time of day in My Castle changed, meaning Scarlet took over the kitchen from Reina, meaning I could actually get some meal buffs for everyone. This makes the earlier part go by smoother and quicker. Also gave Lucina a HP Tonic so she could be more aggressive.
Everything is set up perfectly for Arthur and Effie to rendezvous, and I assume they would retreat after that.
So what happens on the Ally Phase?
Despite there being no Wyvern Riders within the path between them (which was a literal corner space), they decided to WALK AWAY FROM EACH OTHER, engage the Wyvern Riders, that they are WEAK TO, said Wyvern Riders happen to be on adjacent spaces so they counter with a Dual Attack, which kills Effie and Arthur in two separate fights.
Also that was the first time they ever engaged a fight in all my attempts. Normally they run away.
That's not difficulty, that's terrible design at this point. Like what do you even get for saving them?