Alm and Celica is the Prologue of Fire Emblem Echoes: Shadows of Valentia. Set several years before the beginning of the story proper, it details the circumstances that led to Celica's departure from Ram Village.
The chapter consists of a single battle that pits Mycen and the children of Ram Village against a group of Zofian knights led by Slayde. Mycen is capable of defeating any enemy in this battle in two hits, and he is fast enough to double any enemy while also having enough Defense to take only one damage from any attacks that the enemy lands on him. You can easily end this map in a single action by simply attacking Slayde with Mycen right away. However, Slayde is standing on a gravestone, which offers a hefty boost to his Avoid, so hitting him may prove tricky. Slayde will move starting from turn 3 however, so if nothing else, you can simply back off and wait for him to come to you.
During this battle, you also have control of Alm, Celica, Gray, Tobin, Kliff, and Faye, but they are all in the very weak Boy/Girl classes, and are incapable of dealing much damage, while they also take heavy damage from any enemy attack. Luckily, the enemies will prioritize attacking Mycen if he is in their range, so keeping the children safe is easy. No EXP is awarded to any units during this map, so there is no downside to soloing the battle with Mycen.