After a strange dream battle, Corrin awakens in Nohr and must have a practice battle.
Defeat the Boss
The victory condition for this battle is to defeat the boss. Move toward the boss and attack when you get close enough. The boss will not do anything during the Enemy Phase, but will attack you whenever you attack during the Player Phase.
After your first attack, your opponent will create a Healtile. Move your unit onto it and choose Wait to heal up. You only need to do this once, even though it will not heal you up completely.
After using the Healtile once, go up and hit Xander again.
You don't have to heal up before the third time you hit Xander. The combat forecast will tell you that you are likely to die, BUT so is Xander, and since you will be attacking first, you will win. (The combat forecast says that there is a 99% chance that you will successfully hit Xander, but if you don't want to risk that 1% chance of missing, you can use the Healtile again first.)