(Does not move)
Strategies for all difficulties
If you stressed out during the previous two missions, you can relax slightly here, as this is quite possibly the simplest mission of the lot–one you can complete in a single turn if well prepared!
Before you begin, make sure Chrom is well trained and has the Exalted Falchion and the Dual Strike+ skill equipped.
Additionally, ensure he has a strong support partner (A or S Rank) with a Brave weapon equipped (forged with +5 Might if possible) and the Galeforce skill; this can include a lot of people, such as a female Avatar, Lucina with the right mother, etc. Plus a bunch of units who can use the Rescue staff.
As soon as the battle begins, the NPC children will be reduced to 1 HP. You can heal or protect them if you want, but it’s unnecessary if you’re aiming for one turn completion.
First-off, pair Chrom with his partner and have a Galeforce unit/pair (or two) clear out the northern enemies to create a path.
Next, either have a staff wielder move north and cast Rescue on another staff wielder with Rescue or a Dancer. Have the second staff wielder move north and Rescue Chrom’s pair or Dance for the initial staff wielder and have them Rescue Chrom’s pair after moving north a second time.
Have Chrom’s partner kill the Sorcerer and then, after Galeforce activates, have his partner face the boss. Assuming both units are strong enough, you should K.O. the boss in a round of battle, thanks to Chrom’s 3-4 Dual Strikes.
If the boss survives, you may need to plan further Rescue chains, for example to bring a Dancer or Lucina with the Parallel Falchion into the fray.
If the one turn strategy sounds too cheap for you, you’re welcome to play the mission “normally”, but be prepared to face an onslaught of reinforcements from the sigils and enjoy protecting the four NPC kids from harm.
Ideally you’ll want two groups of units–one marching north to deal with the boss and another to protect the kids down south. Chances, are if your units are strong and Galeforce-ready, you’ll reach and demolish the boss in a few turns anyway, ending the mission.