After a strange dream battle, Corrin awakens in Nohr and must have a practice battle.
Defeat the Boss
Move toward Xander, then attack him when you are close enough. He won't attack during the subsequent Enemy Phase; instead, he will create a Healtile behind you. You can move Corrin onto the Healtile to regain health.
Depending on your stats and the level of difficulty you are playing, it might be possible to defeat Xander without using the Healtile, but if there is a chance that Xander could kill you on this turn (i.e. if Corrin has a less than 100% chance to hit, and Xander's Atk is higher than Corrin's current HP), you should use the Healtile to avoid a Game Over.
Xander never attacks during this battle, so keep alternating between attacking and using the Healtile until you have defeated Xander.