In chapter 6, I used Azura who, being a Songstress, was incredibly weak to kill all the Nohrian royals and the one fodder enemy. This gave her a ton of exp early on so she'll be able to hold her own in later maps.
This was incredibly hard because I had to make sure neither Xander nor Ryoma killed each other and the Nohrians are tough so I had to weaken them enough without killing them so Azura can defeat them while also making sure none of my units die. Very difficult. Azura has really good growths, though, so it was worth it. She's going to become strong.
I think this was only possible in Normal difficulty.
Now I just need to have Ryoma kill Xander since no one else can damage him.