This worked for me

In this team, Chrom has Fury 3, Nino is 4 star, Lucina has Aether for her special, and Ninian has Wings of Mercy 2 along with Fortify Def 2. Your first move should be moving Chrom to the right of the map, one space below the attack ranges of Camus, the Troubador and the Axe Cavalier, then put Ninian on the space below him, Nino on the space left of him, and Lucina on the space Left of Nino. That should end your turn. Next turn Chrom will get the Def buff from Ninian, move him to the attack range of the three horseman, then move Nino to the space below Chrom and Lucina to the space left of Nino. Ninian should not move on this turn. The next turn Chrom will get attacked by the Troubador and the Axe Cavalier, with the buff from Fortify Def, this leaves him with 1 HP after Fury damage. Camus will move to the left bridge of the map. On your turn attack Camus with Nino. Camus will put Nino in Wings of Mercy range, use Ninian to sing for Camus and then attack Camus again with Nino for the kill. Move Chrom back two spaces and have Lucina kill the first Axe Cavalier. Next the Red Mage and the Troubador should attack Lucina and then put her into Wings of Mercy range. Your turn. First attack the Red Mage to gain access to Aether, teleport Ninian to Lucina and sing for her, then with Aether activated, use that to kill the Troubador while gaining back HP. End your turn here, and on the enemy phase the second Axe Cavalier will attack Lucina and Lucina will end up killing the second Axe Cavalier. Congrats, you now have a 4 star Camus

Lucina

Ninian

Nino

Spring Chrom